Plastic sheet or plate with flameproofing means

Thermoplastically re-formable plastic sheets with flameproofing means, characterized by a core layer 1-10 mm thick comprised of polyether sulfone or a similar thermoplastic, which core material has a content of at least 0.5 wt. %, preferably at least 2 wt. %, of a flame-protective additive, such as boron trioxide; and said sheet or plate further characterized by outer layers, firmly adhering to and covering the two faces of the core layer, at least 0.05 mm thick and comprised of a thermoplastic having a lower proportional content of the flame-protective additives than the core layer; are particularly useful for interior structures in passenger conveyances, such as airplanes.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a thermoplastically re-formable plastic 
sheet or plate with flameproofing means, useful for interior structures in 
passenger vehicles, passenger ships, and passenger aircraft, and for 
producing structural elements suitable for installation in such 
conveyances. 
2. Discussion of the Background 
Experience has shown that the time required to evacuate large capacity 
conveyances, such as airplanes, in the case of fire is so lengthy that the 
health and life of the passengers are at hazard. Therefore, materials are 
used in interior structures of such conveyances which do not present 
additional hazards for the passengers. Such materials should exhibit a 
minimal emission of heat, smoke, and toxic gases during fires, and the 
onset of any such emissions should be delayed as long as possible. 
In the past a number of thermoplastic plastic materials such as ABS, PC, or 
PES have been used for interior structures of aircraft. These materials 
release substantial heat immediately after ignition. Moreover, ABS emits 
substantial smoke. Therefore these plastics are no longer acceptable for 
interior structures of aircraft. 
The U.S. Federal Aviation Authority, in coordination with the aircraft 
manufacturers, has established, e.g., the following requirements relating 
to measurement methods and limiting values of parameters, for the testing 
of the suitability of materials for interior structures of aircraft: 
TABLE 
______________________________________ 
Prescribed 
Parameter Measured value 
Units maximum 
______________________________________ 
Heat evolution 
HRR kW/m.sup.2 65 
FAR 25,853 (OSU) 
HR kW-min/m.sup.2 
65 
Density of evolved 
D.sub.s (4 min) 100 
smoke 
Toxic gases CO ppm 3,500 
S0.sub.2, H.sub.2 S 
ppm 100 
______________________________________ 
Thermoplastics modified with flame-protective additives according to DE-A 
38 29 712 (corresponds to U.S. Pat. No. 4,981,895, incorporated herein by 
reference) meet these fire regulations. The additives improve the 
combustion properties in the OSU chamber test. In comparison to ABS, PC, 
and PES, plastics modified with the additives substantially improve the 
fire safety of passengers when said plastics are used in the inner 
structures of passenger conveyances. Accordingly, such modified plastics 
are used in large amounts in the aircraft industry. 
Nonetheless, these modified plastics have certain disadvantages. The impact 
strength (Gardner) of sheets comprised of the modified plastics decreases 
with increasing content of the flame-retardant additives. In addition, the 
HRR values determined in the OSU test still appear to be close to the 
maximum permissible values, particularly in the case of 2-5 mm thick 
sheets, which are particularly important in interior construction. 
Furthermore, some of the additives used, particularly boron trioxide, form 
undesirable deposits or efflorescences or blooms on the surface of the 
sheets. 
SUMMARY OF THE INVENTION 
Accordingly, it is one object of the present invention to provide novel 
plastic materials which pass the OSU test and have high impact strength. 
It is another object of the present invention to provide plastic materials 
which pass the OSU test and which also exhibit sufficient mechanical 
strength to be used as thin sheets. 
It is another object of the present invention to provide plastic materials 
which result in a reduction of the heat evolved in burning as measured by 
the OSU test. 
It is another object of the present invention to provide plastic materials 
which pass the OSU test and exhibit a reduced amount of bloom formation on 
the surface. 
These and other objects, which will become apparent during the following 
detailed description, have been achieved by the inventors' discovery that 
multilayer plastic sheets or plates, comprising: (i) of a core layer 1-10 
mm thick comprising a thermoplastic with a content of at least 0.5 wt. %, 
preferably at least 2 wt. %, based on the weight of the core layer, of a 
flame-protective additive; and (ii) outer layers firmly adhering to and 
covering the two faces of the core layer, which outer layers are at least 
0.05 mm thick and which comprise a thermoplastic having a lower 
proportional content of the flame-protective additives than does the core 
layer; possess excellent combustion and mechanical properties. Preferably 
the outer layers contain no flame-protective additives.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The effect of the outer layers is surprising. It was found, surprisingly, 
that not only were the OSU test results for the present composites, 
comprised of a core layer of plastic with flame-protective additives and 
outer layers of pure plastic, superior to the results for homogeneous 
sheets or plates comprised of the pure plastic, but they were also 
superior to the results for homogeneous sheets or plates of plastic 
containing the additive. The combustion which occurred was controlled, 
with carbonization of the surface, which led to decreased flowing and 
decreased bubble formation under the action of an applied fire. Most 
surprising was the fact that no reduction of impact strength (determined 
according to Gardner) could be detected, as would be expected due to the 
content of flame-protective additives in the present multilayer sheets or 
plates. 
The advantageous properties may be seen from the comparison tests described 
hereinbelow. In the tests, polyether sulfone (PES) was used as the 
thermoplastic, and boron trioxide was used as the flame-protective 
additive. 
The comparison tests were carried out on the following materials: 
A) Single-layer extruded sheet comprised of pure PES; 
B) Single-layer extruded sheet comprised of PES with 3 wt. % B.sub.2 
O.sub.3 +8 wt. % TiO.sub.2 (according to DE-A 38 29 712); and 
C) Three-layer co-extruded sheet with a core layer comprised of a plastic 
according to (B) and outer layers adhering to said core layer on both 
faces of the latter, which outer layers are comprised of pure PES. 
TABLE 1 
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Heat release rate (HRR) according to FAR 25,853 (OSU test): 
HRR (kW/m.sup.2) 
Sheet thickness (mm): 
Material 1.5 2.0 2.5 3.0 
______________________________________ 
A 56 65 78 88 
B 37 48 53 56 
C 41 47 50 51 
______________________________________ 
TABLE 2 
______________________________________ 
Gardner impact strength test: 
Sheet thickness (mm): 
Material 1.5 2.0 2.5 3.0 
______________________________________ 
A &gt;18 &gt;18 &gt;18 &gt;18 
B 8 10 14 16 
C &gt;18 &gt;18 &gt;18 &gt;18 
______________________________________ 
The outer layers with low or preferably zero content of additives (e.g., 
boron trioxide) serve to effectively prevent efflorescence. 
The flameproofed sheets or plates according to the invention are 
advantageously produced by techniques of multilayer co-extrusion in which 
at least 3 layers are co-extruded. These techniques are per se known. The 
core layer may itself be comprised of a plurality of layers. 
Preferably the thickness of the sheets or plates is 0.5-20 mm, particularly 
preferably 0.8-5 mm. Sheets in the range 1-3 mm thick are of particular 
practical interest for aircraft construction. 
The outer layers may be 0.05-2 mm thick, preferably 0.1-1.0 mm. Outer 
layers &lt;0.1 mm thick have been found to be relatively ineffective in many 
cases, with the combustion behavior corresponding to the behavior of the 
pure core layer. A reduction of the OSU test value is always observed with 
outer layers at least 0.1 mm thick each, and particularly at least 0.15 mm 
thick. OSU test values decrease further as the thickness of the outer 
layer increases, up to an outer layer thickness of 0.25 mm. 
The thermoplastic of which the core and outer layers are comprised should 
be extrudable, and should be susceptible to thermoplastic and/or 
thermoelastic deformation of the extruded sheets or plates comprised of 
the material. It should have high thermal stability. Advantageously, its 
Vicat softening temperature is in the range 150.degree.-250.degree. C. The 
extended use temperature of the material (according to UL 746 B) should be 
&gt;130.degree. C., preferably &gt;150.degree. C. In the case of fire, it should 
have low smoke emission density, and should form minimal or no toxic 
combustion gases. These requirements are met by a number of engineering 
thermoplastics which have as a common feature aromatic groups, 
particularly phenylene groups, in their main chain. Known representatives 
of this group of thermoplastics are polyether sulfones (PES), polyether 
imides (PEI), polyaryl ether ether ketones (PEEK), polyphenylene sulfides 
(PPS), thermoplastic polyimides (PI), polyamideimides (PAI), and 
self-reinforcing liquid crystal polymers (LCP). 
Short descriptions of these plastics and methods by which they can be 
produced are found in Franck, A., and Biederbick, K., 
Kunststoff-Kompendium, 2nd, Ed., pub. Vogel Buchverlag, Wurzberg (1988). 
Typical structures of such plastics are presented in the formulae shown 
below: 
##STR1## 
Preferred plastics are polyether sulfones. These are to be understood as a 
class of polycondensate plastics belonging to the polysulfones, the 
members of which are comprised of bifunctional aromatic groups, 
particularly phenylene groups, joined by ether and sulfone bridges. In the 
following, these are designated as "PES", in accordance with DIN 7728 T. 
PES are thermoplastic, thermally stabile, and self-extinguishing. For the 
purposes of the invention, PES with a melt viscosity suitable for 
extrusion are employed; the melt volume index at 360.degree. C./10Kp is, 
e.g., approximately 30 cm.sup.3 /10 min. 
The preferred plastics are comprised of recurring units of general 
structural formula 
EQU --(Ar--SO.sub.2 --Ar--O).sub.n --, 
where Ar represents a bifunctional mononuclear or polynuclear aromatic 
group, chosen such that the required melt viscosity is achieved. 
Preferably the groups Ar comprise p-phenylene groups, which may contain 
substituents such as lower alkyl groups or cycloalkyl groups. Polynuclear 
groups Ar may contain, e.g., two such phenyl groups which may be directly 
connected or may be linked via an oxygen or sulfur atom or by --SO.sub.2 
--, methylene, or isopropylidene groups. 
The core and outer layers may be comprised of different plastics. However, 
it is advantageous if the outer layers contain the same thermoplastic 
plastic as the core layer, except for the content of flame-protective 
additives. As a rule the outer layers have zero content of 
flame-protective additives. There may be a small amount of such additives, 
not greater than 0.5 wt. %, e.g., if the additives diffuse to a small 
extent from the core into the outer layers under the conditions of 
co-extrusion, or if a small amount of recycled sheet or plate material is 
used in producing the outer layers. 
The most favorable mechanical properties are achieved when the outer layers 
are completely free of flame-protective additives. Where a certain extent 
of degradation of favorable mechanical properties is tolerable, a certain 
amount of flame-protective additives may be incorporated in the outer 
layers. Preferably this amount is much less than 1 wt. %, particularly 
preferably &lt;0.5 wt. %, based on the weight of the outer layers. 
The core layer contains an effective amount of flame-protective additive, 
which amount is determined such that the multilayer sheet or plate passes 
the OSU combustion test. The amount of additives is at least 0.5 wt. %, 
preferably at least 2 wt. %, particularly preferably 2.5-15 wt. %, based 
on the weight of the core layer. 
Flame-Protective Additives 
Within the context of the invention, all materials which inhibit heat 
evolution under combustion conditions and which do not liberate toxic 
gases or vapors upon burning are regarded as "flame-protective additives". 
Particularly suitable additives are inorganic materials which when cooled 
from a molten state undergo a transition to an amorphous vitreous state 
and in the process form a planar network structure or three-dimensional 
network structure. The molten plastic is wetted and is coated with a layer 
of decomposed additive material. 
The additives should have a melting point between 300.degree. and 
1400.degree. C., preferably between 350.degree. and 1200.degree. C., 
particularly preferably between 400.degree. and 1000.degree. C., or else 
when subjected to fire should be converted into materials with such a 
melting point. Examples of such additives are oxides of elements of groups 
III-B to V-B of the periodic table, and salts of oxygen acids of such 
elements. Preferred salts are Na, K, Ca, Zn, and Al salts. Hydroxides and 
carbonates of the mentioned elements may be converted into oxides with the 
prescribed properties, under fire conditions. Also, mixtures of a 
plurality of additives may be employed. 
Examples of suitable additives are B.sub.2 O.sub.3, NaBO.sub.2, Na.sub.2 
B.sub.4 O.sub.7, Ca(BO.sub.2).sub.2, Zn(BO.sub.2).sub.2, Zn.sub.2 B.sub.2 
O.sub.11, P.sub.2 O.sub.5, Na.sub.4 P.sub.2 O.sub.7, NaPO.sub.3, 
Ca(PO.sub.3).sub.2, Zn.sub.3 (PO.sub.4).sub.2, Sb.sub.2 O.sub.3, Sb.sub.2 
O.sub.5, Na.sub.3 SbO.sub.4, Na.sub.2 SiO.sub.3, Na.sub.2 
O-CaO-6SiO.sub.2, K.sub.2 O-Al.sub.2 O.sub.3 -6SiO.sub.2 (feldspar), 
Na.sub.2 O-Al.sub.2 O.sub.3 -6SiO.sub.2 (zeolite), and other silicates. 
Other additives may be present in the outer layers and the core layer, 
which further reduce the HRR values; e.g., magnesium hydroxide. In 
addition, the combustion properties are favorably influenced by the 
presence of titanium dioxide in the outer and core layers, e.g., in an 
amount of 5-10 wt. %. 
A preferred flame-protective additive, particularly for addition to PES, is 
boron trioxide. If the core layer is at most 2 mm thick, a content of 2 
wt. % boron trioxide is sufficient. If the core layer is &gt;2 mm thick, a 
content of boron trioxide of at least 2.5 wt. %, e.g., 3 wt. %, is 
advantageous. Preferably the core layer also contains titanium dioxide. 
The outer layers may contain 1-12 wt. % of a white pigment. 
The novel sheets and plates may advantageously be manufactured by 
co-extrusion of the three layers. The surface of the extruded sheet may be 
smoothed or textured in a calendering apparatus. To produce formed pieces, 
the extruded planar sheets or plates may be heated to softening and may be 
formed by vacuum and/or a drawing punch, on a forming apparatus. 
Other features of the invention will become apparent in the course of the 
following descriptions of exemplary embodiments which are given for 
illustration of the invention and are not intended to be limiting thereof. 
EXAMPLES 
Example 1 
In two separate extruders, 
-- PES (commercial product Ultrason.RTM. E 3000 Q 12, of BASF AG) with a 
content of 3 wt. % B.sub.2 O.sub.3 and 8 wt. % TiO.sub.2, along with 
pigments, for the core layer, and 
-- pure (other than a small amount of pigments) PES, for the outer layers, 
were melted, heated to 350.degree. C., and were combined into a 
three-layered intermediate extrudate with the use of a co-extrusion 
adapter. This extrudate was further extruded to a 1.5-3 mm thick sheet, 
using an adjustable slit nozzle. The sheet was fed to a calender where it 
was smoothed or impressed on one or both sides, and the sheet was cooled 
to below 226.degree. C. The two outer layers of the sheet were each 0.2 mm 
thick, and the core layer between them was 1.1-2.6 mm thick. The sheet was 
cut into sheet goods of conventional commercial size, using a cutting 
apparatus. 
The fire behavior in the OSU test in comparison to single-layer sheets 
comprised of pure PES and sheets comprised of the above-described mixture 
is given in tabular form hereinabove in the Specification. 
Example 2 
Using the method described in Example 1, 3 mm thick sheets were produced by 
co-extrusion, which sheets had a core layer of PES with a content of 3 wt. 
% B.sub.2 O.sub.3 and 8 wt. % TiO.sub.2, and some of which had co-extruded 
outer layers comprised of pure PES. The following values were obtained in 
the OSU test, each of which represents the mean of 5 individual 
measurements: 
______________________________________ 
Thickness of the 
OSU test results: 
outer layers (mm) 
HRR (kW/m.sup.2) 
HR (kW-min/m.sup.2) 
______________________________________ 
0.0 60-65 &lt;10 
0.06-0.08 71 4 
0.10-0.12 37 3 
0.15-0.17 47 7 
0.20-0.22 49 9 
______________________________________ 
If for the outer layers one uses PES which also contains 8 wt. % TiO.sub.2 
one obtains the following OSU test results: 
______________________________________ 
Thickness of the 
OSU test results: 
outer layers (mm) 
HRR (kW/m.sup.2) 
HR (kW-min/m.sup.2) 
______________________________________ 
0.0 60-65 &lt;10 
0.06-0.08 66 3 
0.10-0.12 59 4 
0.15-0.17 48 5 
0.20-0.22 45 10 
______________________________________ 
Obviously, numerous modifications and variations of the present invention 
are possible in light of the above teachings. It is therefore to be 
understood that, within the scope of the appended claims, the invention 
may be practiced otherwise than as specifically described herein.