Stock material or miscellaneous articles – Structurally defined web or sheet – Including grain – strips – or filamentary elements in...
Reexamination Certificate
1997-12-22
2001-02-06
Raimund, Christopher (Department: 1771)
Stock material or miscellaneous articles
Structurally defined web or sheet
Including grain, strips, or filamentary elements in...
C428S112000, C428S219000, C264S241000, C264S250000, C264S263000
Reexamination Certificate
active
06183834
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a ballistic-resistant moulded article containing a compressed stack of monolayers, with each monolayer containing unidirectionally oriented reinforcing fibres and at most 30 wt. % of a plastic matrix material and with the fibre direction in each monolayer being rotated with respect to the fibre direction in an adjacent monolayer. The invention also relates to a process for the manufacture of the moulded article. The ballistic-resistant moulded article may be used in, for instance, helmets, as inserts in bullet-proof vests, as armouring on military vehicles and in ballistic-resistant panels.
BACKGROUND OF THE INVENTION
A ballistic-resistant moulded article of this type is disclosed in WO 89/06190. The known ballistic-resistant moulded article affords fairly good protection against impacts of projectiles such as shrapnel or bullets. The level of protection is quantified by means of the Specific Energy Absorption (SEA), a measure of the amount of energy that can be absorbed by a moulded article on impact of a projectile per unit areal weight of the moulded article. Fairly good protection by the known moulded article on impact of a bullet is intended to mean an SEA value of from about 30 Jm
2
/kg to maximum 65 Jm
2
/kg. The March 1992 “Ballistics” leaflet of Messrs Allied-Signal Inc., Petersburg, Va., quotes as highest SEA level an SEA of 65 Jm
2
/kg on impact of a 7.62×39 Mild Steel Core P.S. Ball M1943. This SEA level has been achieved after years of optimization and was regarded by ballistics experts as being very high.
Nevertheless, there continues to be a great need for ballistic-resistant moulded articles that can offer increased protection against impacts of projectiles of various kinds, especially against projectiles in the form of bullets.
SUMMARY OF THE INVENTION
The inventors of the present invention have found that this object is achieved by the density &rgr;
P
of the compressed stack in the ballistic-resistant moulded article being at least 98.0% of the theoretical maximum density.
The theoretical maximum density &rgr;
th
is calculated using the following formula:
&rgr;
th
=&rgr;
f
m
f
+&rgr;
k
m
k
where
&rgr;
th
=density of the fibres
&rgr;
k
=density of the plastic
m
f
=mass fraction of the fibres in the compressed stack
M
k
=mass fraction of the plastic in the compressed stack
Surprisingly, a ballistic-resistant moulded article of the invention has an SEA value of at least 75 Jm
2
/kg on impact of a 7.62×39 Mild Steel Core P.S. Ball M1943.
The percentage of the theoretical maximum density is hereafter referred to as the relative density. Preferably, the relative density of the compressed stack is at least 98.5%, more preferably at least 99.0% and most preferably at least 99.5%. In this way, a ballistic-resistant moulded article having an even better SEA value is obtained. More preferably, the SEA value is at least 90, even more preferably at least 110 Jm
2
/kg, most preferably at least 120 Jm
2
/kg.
The density of the ballistic-resistant moulded article can be determined in a manner known to one skilled in the art. A suitable, accurate method is described in ASTM D 792-91. Another suitable method for flat or slightly flexed moulded articles comprises measurement of the density by measuring the weight and accurately measuring the volume with the aid of a Vernier calliper. By this measurement, if accurately performed, the density can be determined with a standard deviation of 0.2-0.4%.
The SEA is defined by 0.5.m.v
50
2
/AD, where m is the mass of the projectile. The v
50
is the velocity of the projectiles fired at the moulded article at which 50% of the projectiles fully penetrate the moulded article, AD is the areal weight of the moulded article.
Owing to the surprisingly high SEA achieved, not only moulded articles with an unheard-of level of protection for a given weight of the moulded article are now available but also moulded articles affording the same level of protection as the known moulded article at a significantly lower weight. Low weight per unit area is of great importance in many applications. This is the case, for instance, in the field of personal protective equipment such as helmets, shields, shoes and the like. Low weight is also essential for the application of ballistic-resistant moulded articles in for instance helicopters, motorcars and high-speed, highly manoeuvrable combat vehicles.
The areal weight of the moulded article depends on the desired level of protection. Preferably, the areal weight is between 10 and 40 kg/m
2
. Below 10 kg/m
2
, the level of protection is inadequate for most threats, above 40 kg/m
2
the weight will often be too high, especially for body protection.
In the context of the present application “monolayer” means a layer of substantially parallel reinforcing fibres embedded in a plastic matrix material. The term “matrix material” means a material which binds the fibres together and which wholly or partially encapsulates the fibres. Such monolayers (usually called prepregs by one skilled in the art) and methods of obtaining such a monolayer are disclosed in for instance EP-B-0.191.306 and WO 95/00318. A monolayer may be obtained by for instance pulling a number of fibres from a fibre bobbin frame over a comb so that they are oriented in coplanar and parallel fashion in one plane and then impregnating the fibres with the plastic. In this process, fibres may be used that have previously been coated with a polymer other than the plastic matrix material in order to, for instance, protect the fibres during handling or in order to obtain better adhesion of the fibres onto the plastic of the monolayer. Preferably, uncoated fibres are used. Impregnation of the reinforcing fibres with the plastic matrix material can be effected by applying one or more films of the plastic to the top, bottom or both sides of the plane of the fibres and then passing these, together with the fibres, through a heated pressure roll. Preferably, however, the fibres, after being oriented in coplanar, parallel fashion in one plane, are coated with an amount of a liquid substance containing the plastic matrix material of the monolayer. The advantage of this is that more rapid and better impregnation of the fibres is achieved. The liquid substance may be for example a solution, a dispersion or a melt of the plastic. If a solution or a dispersion of the plastic is used in the manufacture of the monolayer, the process also comprises evaporating the solvent or dispersant.
“Reinforcing fibre” here means an elongate body whose length dimension is greater than the transverse dimensions of width and thickness. The term “reinforcing fibre” includes a monofilament, a multifilament yarn, a tape, a strip, a thread, a staple fibre yarn and other elongate objects having a regular or irregular cross-section. Any natural or synthetic fibre may in principle be used as reinforcing fibre. Use may be made of for instance metal fibres, semi-metal fibres, inorganic fibres, organic fibres or mixtures thereof. For application of the fibres in ballistic-resistant moulded parts it is essential that the fibres be ballistically effective, which, more specifically, requires that they have a high tensile strength, a high tensile modulus and/or high energy absorption. It is preferred for the fibres to have a tensile strength of at least 1.2 GPa and a tensile modulus of at least 40 GPa.
Suitable inorganic fibres having a high tensile strength are for example glass fibres, carbon fibres and ceramic fibres. Suitable organic fibres having a high tensile strength are for example aramid fibres, liquid crystalline polymer fibres and fibres of for instance polyolefins, polyvinylalcohol, polyacrylonitrile which are highly oriented such as those obtained by means of a gel spinning process.
An extensive enumeration and description of fibres that may be used in the monolayer are given in WO 91/12136 (line 23 on page 6 to line 8 on page 12).
Homopolymers and copolymers of polyethylene and p
DSM N.V.
Pillsbury Madison & Sutro LLP
Raimund Christopher
LandOfFree
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