Reversible agglomeration of fine particle plaster of Paris

Details Reversible agglomeration of fine particle plaster of Paris Reversible agglomeration of fine particle plaster of Paris

1999-07-02

2001-01-09

Green, Anthony (Department: 1755)

Compositions: coating or plastic

Coating or plastic compositions

Inorganic settable ingredient containing

C106S778000, C106S783000, C106S784000, C106S785000, C106S461000, C423S555000

Reexamination Certificate

active

06171387

ABSTRACT:

This invention relates to a method for the reversible agglomeration of fine particle plaster of Paris, to the reversible agglomerates made by the method, to precursor mixtures for making aqueous compositions containing the reversible agglomerates plus water and to the compositions themselves. Plaster of Paris is the common name for calcium sulphate hemihydrate or CaSO
4
.½H
2
O, also known as casting plaster.
Water hardens plaster of Paris by hydrating it from the hemihydrate to the dihydrate which is gypsum or CaSO
4
.2H
2
O. The hydration is accompanied by a small expansion which enables plaster of Paris to find use in aqueous moulding and filler compositions, sometimes called “hydraulic setting pastes”. More specifically, moulding compositions may be pastes or slurries of plaster and water, whilst filler compositions are usually pastes. Slurries may also be used to impregnate cloth so that when the plaster hardens, a rigid structure is created as is the case when plaster of Paris is used to make splints for broken bones.
Such uses of plaster of Paris require the efficient dispersion of particles of plaster in water in a way which ensures that the particles are thoroughly wetted. The dispersion is often performed by hand and (at least in the case of fillers) by unskilled users such as DIY (do-it-yourself) householders. Efficient dispersion in such circumstances demands the use of fine particles, preferably having a weight average particle dimension of below 100 &mgr;m, where 1 &mgr;m equals 1×10
−6
m. Unfortunately, the delivery of particles of this fineness from a packet to the water into which they are to be dispersed can create various problems especially for unskilled users. Firstly, fine particles of plaster of Paris have a poor ability to flow and so they (and dry compositions containing them) do not pour easily from a packet. This often leads the user to tap the packet or to use a spoon. In addition to creating a dust, tapping the packet also often causes a sudden delivery of a large volume of plaster into the water and of course spooning will also deliver a large volume in a single amount. Such large volumes of plaster will have a relatively low ratio of easily accessible surface area to volume of plaster which means that the wetting of the plaster will be very inefficient. As a result, the plaster will clump into large irregularly sized pellets containing unwet particles.
Large irregular sized pellets formed by clumping need to be broken down during the subsequent dispersion of the plaster into the water for if they survive into the final paste or slurry, they will be a source of defects in the surface of the hardened plaster. For example, they will form blemishes in a plaster caste or in the surface formed by a hardened filler composition which has been applied by a filling knife or trowel into a hole or crack in a wall or ceiling. The breaking down of the pellets demands the exertion of significant shear which is difficult and tiring to apply when dispersing by hand. Many DIY users do not have the sustained strength or patience to achieve a sufficient break down and so they become dissatisfied with the surfaces they achieve.
Clearly it would be desirable to improve the flow of fine particulate plaster of Paris without inhibiting the ability of its particles to be wetted by water. Unfortunately, the use of binder additives to coarsen the plaster usually increases the time needed to disperse the plaster in water and may inhibit the ability of the plaster to harden.
This invention provides a method for the reversible agglomeration of fine particle plaster of Paris (which preferably comprises particles having a weight average particle dimension of less than 300 &mgr;m and especially less than 100 &mgr;m) which method includes
a) providing a vessel which in turn is provided with means capable of agitating fine particles of plaster of Paris,
b) introducing into the vessel both fine particles of plaster of Paris and an organic polar liquid,
c) subjecting the particles and the liquid to agitation and
d) subsequent to a period of agitation, subjecting the plaster of Paris to drying to remove at least a major proportion (preferably at least 95 wt %) of the organic polar liquid
whereby pluralities of particles spontaneously interlink to form agglomerates having a weight average dimension in excess of 100 &mgr;m and preferably in excess of 300 &mgr;m and in which interlinks are disruptable when the agglomerates are dispersed by hand into water. Disruption of the agglomerates reverses the agglomeration process and so releases fine particles into the water. It can easily be accomplished by stirring the agglomerates by hand into water using a spoon, knife, trowel or other stirrer whereupon the particulate plaster of Paris becomes efficiently wetted. Agglomerates of as much as 10 mm maximum dimension have been achieved, but a maximum dimension range of 0.2 to 2 mm (especially 0.3 to 1 mm) is preferred. The dimensions of the agglomerates can be adjusted by adjusting the amount of agitation to which the particles of plaster of Paris are subjected.
The purpose of agitating the particles and polar liquid is to ensure that the two are efficiently mixed together (either alone or with any optional auxiliary additives) so that the liquid can thoroughly wet the surfaces of the particles of plaster and any other additives present. Any agitating means which can achieve this will be suitable. A convenient means comprises a rotary blade impeller-type mixer in which the blades are located near the base of a cylindrical vessel which houses them. A particularly suitable mixer is supplied by Zanchatta & C s.r.l of San Salvatore, Lucca, Italy and described by them in their brochure “Roto Technology”, the contents of which are herein incorporated by reference. The mixer comprises three co-planar equally spaced radial blades which make a running fit in a vessel. The blades are about 14 cm long and are rotated at a speed of from 100 to 800 rpm or more preferably from 400 to 600 rpm.
It is convenient to use a fine particle plaster of Paris in which 30 to 60 wt % of the particles of plaster have a weight average particle dimension of less than 50 &mgr;m and preferably 30 to 50 &mgr;m. Such plaster is sold as “fine casting plaster”.
It is very much preferred that the organic polar liquid should have at least some ability to form hydrogen bonds even though it is by no means certain that hydrogen bond formation plays any part in the successful performance of this invention. Preferred liquids are hydroxy compounds and in particular the straight chain aliphatic monohydric alcohols containing up to 4 carbon atoms. Methanol tends to form agglomerates which are to some extent more difficult to disrupt by stirring in water whereas butanol and isopropanol form agglomerates which are less robust being only moderately resistant to disruption by dry shearing forces of the type met during the filling or transport of packets of plaster. Ethanol and n-propanol both produce robust agglomerates which nevertheless disrupt easily on stirring in water with n-propanol producing the best balance between robustness and disruptability in water.
It is preferred to use the organic polar liquid in an amount which is between the minimum needed to wet completely the surfaces of the particles and the amount which causes the particles to have the flow characteristics of a slurry. For this reason, it is preferred to use from 7 to 14 wt % of the liquid where the percentage is based on the combined weights of the particles and the liquid. It is particularly preferred to use from 10 to 14 wt % of n-propanol or ethanol.
The use of over-wet alcohols can sometimes cause premature hydration of the plaster which in turn can hinder or prevent the disruption of the agglomerates leading to poorer wetting or in extreme cases to the formation of fully hardened particles of gypsum. Small amounts of water can be tolerated particularly in n-propanol, but it is preferred to have less than 0.3 wt % present. Organic liquids containi

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