Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
1999-10-04
2002-02-19
Cain, Edward J. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C106S415000, C106S469000
Reexamination Certificate
active
06348536
ABSTRACT:
FIELD OF THE INVENTION
The invention concerns a process for treating a lamellar type mineral (i.e. of which the structure consists of a stack of elementary leaves), with a view to obtaining a powder with a high lamellarity. It concerns in particular the manufacture of talc, kaolin or mica powders, having at the same time a fine or very fine particle size and a high lamellarity. It extends to applications for the said powders as functional fillers for thermoplastic materials with a view to increasing the modulus of elasticity in flexure of these materials for a given impact resistance. “Functional filler” is understood to mean an additive incorporated in the thermoplastic material with a view to increasing its performances (as against a “passive filler” which modifies the properties of the plastic material obtained very little and essentially serves to reduce the cost).
BACKGROUND OF THE INVENTION
It is now current practice to incorporate mineral fillers into thermoplastic materials in the form of powders, such as talc, kaolin or mica powders with a view to increasing the modulus of elasticity in flexure. (This modulus of elasticity in flexure is defined by the “ISO 178” standard which describes the procedure for measuring this modulus. All the measurements of the said modulus which are provided hereinafter were carried out according to this standard.) This increase in the modulus of elasticity in flexure of the material obtained enables the dimensions of parts manufactured from the said material to be reduced and hence to be made lighter for a given rigidity. This reduction in weight is essential in several sectors of industry, in particular in the automobile sector and the packaging sector and it also leads to a reduction in costs.
At the present time, mineral powders which are incorporated in thermoplastic materials are fine powders. For talc in particular, the mean particle size is generally between 2 and 10 microns and is obtained by dry grinding and selection, in particular in installations of the “air or steam micronizer” type where the mineral is ground and selected dry. In order to increase the value of the modulus of elasticity in flexure still further, it is proposed in several publications to reduce the particle size of the powders used (cf. for example “W. Hobenberger, Fillers, Kunststoffe plast Europe 86, Jul. 7, 1996, pp 973-977”; “De Wilhelm Schober, Talc for thermoplastics, Industrial minerals, May 1995, pp 49-53”). The consistent idea of associating the modulus of elasticity in flexure of the final material obtained with the particle size of the filler is currently generalized in the field of functional fillers for thermoplastics, and those working in the field are engaged in improving the dry grinding and selection techniques in order to obtain increasingly fine powders (the dry grinding and selection techniques being in point of fact known for the purpose of obtaining fine particle sizes industrially).
Another type of grinding also exists, known as wet grinding, consisting of suspending the mineral in a liquid and of stirring it in the presence of a ball milling charge. This wet grinding technique is generally used for grinding and homogenizing paints or for grinding mineral fillers in the paper industry. For example, WO/9712002 and WO/9624639 describe pigments for paper pulp having been subjected to wet grinding which enables the optical properties of the papers obtained to be improved. The grinding efficiency is measured by the D.I. obtained (“Delamination Index”: difference between a particle size characteristic of the finished product and a particle size characteristic of the product subject to grinding). It should be emphasized that this DI index does not intrinsically characterize a powder but an operation used, in terms of particle size.
Studies have been carried out in the field of thermoplastic materials to evaluate the value of using this type of wet grinding technique for functional mineral fillers to be incorporated in plastic materials, but these studies have resulted in a negative conclusion. Although the modulus of elasticity in flexure may be increased (for an equivalent particle size) by using wet grinding, this wet grinding technique leads on the other hand to an unacceptable defect in the manufacture of plastic materials, namely a considerable reduction in the impact resistance of the plastic material obtained. (The impact resistance referred to in all the following text is the un-notched CHARPY impact resistance at −20° C. measured according to the “ISO 179” standard). Reference may for example be made to the following publication which refers to these wet grinding tests for manufacturing mineral fillers for thermoplastic materials: “L. J. Michot et al., Journ. Mater. Sci. (1993), 28 (7) pp 1856-66”.
The state of the prior art in the field of functional mineral fillers which can be incorporated in thermoplastic materials is therefore currently as follows:
on the one hand, wet grinding has never been used industrially, taking into account the problems
of impact resistance which it entails (except for fillers essentially based on mica with a view to producing parts which require only very low impact resistances),
on the other hand, the route taken by industrial development in order to increase the modulus of elasticity in flexure of the final material consists of refining the particle size of mineral powders used as fillers, by improving the dry grinding techniques for these powders.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a novel mineral powder, in particular talc, kaolin or mica, suitable for incorporation as a functional filler in a thermoplastic material with a view to increasing the modulus of elasticity in flexure thereof in significant proportions higher than that obtained with known fillers and this without reducing the impact resistance of the final material (compared with the values obtained with the said known fillers).
“Powder” is understood to mean an uncompacted product in which the particles are free to move in relation to each other, as well as a densified product where the particles or certain particles are temporarily bound in agglomerates.
The mineral powder to which the invention relates is in particular a talc, kaolin or mica powder, and is characterized by:
a particle size distribution such that the median diameter of the particles D
50
lies substantially between 0.5 and 5 microns, the cutoff diameter D
95
is less than 10 microns, and the cutoff diameter D
98
is less than 20 microns,
a specific surface area (BET) greater than 10 m
2
/g,
a high lamellarity index greater than 2.8 and preferably greater than 4.
“Median diameter D
50
” is understood to mean a diameter such that 50% of the particles by weight have a size less than the said diameter; “cutoff diameter D
95
” is understood to mean a diameter such that 95% by weight of the particles have a size less than the said diameter; “cutoff diameter D
98
” is understood to mean a diameter such that 98% by weight of the particles have a size less than the said diameter. For non-spherical particles, the size consists of the equivalent spherical diameter (Stocks diameter). All measurements of the diameters D
50
, D
95
, D
98
were carried out by means of a “Sedigraph” apparatus (trade name) by gravity sedimentation in accordance with standard AFNOR X11-683.
“Specific surface area (BET)” is understood to mean the area of the surface of the particles of the powder with respect to unit mass, determined according to the BET method by the quantity of argon adsorbed on the surface of the said particles so as to form a monomolecular layer completely covering the said surface (measurement according to the BET method, AFNOR standard X 11-621 and 622).
“The lamellarity index” characterizes the shape of the particle, and more particularly its flatness (large dimension/thickness). In all the following, this lamellarity index will be measured by the difference between, on the one hand, the value of the mean dimension of the particles of the p
Fourty Georges
Jouffret Frederic
Monnot Patrice
Cain Edward J.
Talc de Luzenac
Young & Thompson
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