Compositions: coating or plastic – Coating or plastic compositions – Inorganic settable ingredient containing
Patent
1996-12-31
1998-09-29
Green, Anthony
Compositions: coating or plastic
Coating or plastic compositions
Inorganic settable ingredient containing
106634, 106696, 106708, 106724, 106727, 106778, 106781, 106790, 106808, 106822, 106823, 264175, 2641761, 26421111, 264333, 524 4, 524 5, 524 6, C04B 2400, C04B 2102, C04B 2412
Patent
active
058141464
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention concerns a cement material containing polymers, which shows an improved impact strength, meant as an increase in breaking total energy, thanks to fibers addition.
PRIOR ART
The so called MDF (macro defect free) cement, namely free from macroscopic defects, is a kind of cement well known in the art.
The American Patent U.S. Pat. No. 4,070,199 in fact describes an hydraulic material having a high flexural strength, prepared by kneading by means of a cylinder mixer a mixture of hydraulic cement, water and a polymer soluble or dispersable in water.
The product is then pressed and left to harden.
Subsequent Patents (EP 55035 and EP 21682) describe other polymeric materials which can be included in cement mixtures, such as hydroxypropylmethylcellulose or partially hydrolized polyvinyl acetate.
The use of fibers in MDF cement matrices is well known in the art. For instance, Italian Patent No. 1,068,427 reports the use of inorganic mineral fibers (asbestos fibers), organic or polymeric fibers (polypropylene or nylon). According to such a Patent, after addition of nylon fibers cut in 20 deniers pieces having a constant length equal to 10 mm, the hardened cement matrix shows a high proportional elastic limit, which remains substantially equal even when increasing the quantity of fibers up to 6% by weight.
However it is necessary to emphasize that in the whole known art relating to the MDF products, fibers addition to the cement matrix in order to replace the fracture behaviour of the composite with a typically pseudo-ductile stress-strain trend has never been considered. The addition of fibers was instead described in order to increase the mechanical resistance nevertheless preserving a breaking behaviour of fragile kind.
For instance, the patent EP 114,518 teaches a way to improve the fracture energy of these materials by adding randomly distributed fibers in the shape of a fibrous "mat" or in a cloth-shaped oriented way.
However, even if efficacious in order to produce laminated materials, the achievement of cement materials of this kind prevents from producing materials having more complex geometrical shapes, and in any case, does not modify the behavior of the composite material, but rather it only increases its impact strength.
The article "Toughening of MDF Composites" by C. K. Park (Mat. Res. Soc. Symp. Proc. vol. 211, 1991) reports examples of the use of short carbon, glass or cellulose fibers for preparing particular MDF matrices (comprising fly ashes and microsilica) to estimate the impact strength of the resulting materials in the elastic field (K.sub.IC).
The problem that the present invention wants to solve is not that of obtaining a composite material with increased mechanical resistance with respect to the base matrix but that of achieving a composite cement material able to support high loads at high strains even after the fracture propagation inside the composite itself has started. This means to define the achievement of the impact strength of the composites of the invention as the increase in the breaking total energy (the integral of the stress-strain curve) combined with a modification of the breaking behavior of the material from fragile to pseudo-ductile. Therefore we mean to consider the whole stress-strain curve up to the achievement of the maximum load, rather than limiting ourselves just to the part of the curve included in the elastic field.
In the present description and claims, "pseudo-ductile" material means a material which, when stressed by a flexure load, shows an improved post fracture impact strength. To even better explain the problem which the invention solves, one has to keep in mind that a traditional MDF cement matrix gives way once stressed by a load having low strain values, whereas the products of the present invention are such that the fibers therein contained preserve the capability to transfer the stress through the fracture by means of bridge bonds formation and therefore undergo pull out processes with consequent increa
REFERENCES:
patent: 4070199 (1978-01-01), Downing et al.
Toughening of MDF --OPC Composites, Mat. Res. Soc. Symp. Proc. vol. 211. 1991 Materials Research Society.
Abstract of Italian Patent Specification No. 1,068,427 (Mar. 1985).
English version of ENV 197-1: 1992 standard Italian UNI ENV 197, 1.
Franchini Marcello
Guerrini Gianluca
Maggio Rosa Di
Migliaresi Claudio
Poli Stefano
Green Anthony
Italcementi S.p.A.
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