Plastic container for liquids or gases

The invention relates to plastic containers for liquids and gases, which are characterized in that they have a seamless inner layer of modified PPO or PP and a glass fiber layer following it, laid on geodesic lines and provided partly or completely with reaction resins or thermoplastics as well as an optional outer coating of hardenable reaction resins or thermoplastics. In regions of particular strain, the plastic containers may have, between the liner and the glass fiber layer comprising long fibers, an intermediate layer of thermoplastics or reaction resins not having containing monomeric styrene and with approximately 20 to 40% by weight of glass fibers in the form of short fibers.

BACKGROUND OF THE INVENTION 
The invention relates to plastic containers for liquids or gases. 
Composite materials, that is, laminated bodies made of various plastics, 
are already known, and in many fields they have replaced the materials 
previously typically used. However, it has not yet been possible to use 
such composite plastics or sandwich materials for all fields of 
application, because either their mechanical properties or other 
characteristics did not allow their use in place of metal or ceramic 
materials. Among these fields of application are for instance the 
manufacture of water tanks, in particular hot-water heaters or tanks for 
other aqueous liquids, which to a large extent are still today made of 
enameled steel. Enameled steel tanks are markedly vulnerable to corrosion, 
however, because hairline cracks form in the enamel film, and the 
equipment undergoes considerable corrosion relatively quickly at 
temperatures of approximately 601/2.degree. C. or below. Experience has 
shown that such water tanks of steel last for only about 2 to 3 years and 
then must be replaced because of corrosion damage. It is already known 
from German Patent Application No. DE-OS 33 42 386, however, that instead 
of steel tanks, tanks made of plastic can be used; these tanks comprise an 
inner layer of modified polyphenylene oxide, i.e., PPO, and an outer layer 
of glass-fiber-reinforced polyester resin. The glass-fiber-reinforced 
polyester resins have a glass fiber content of approximately 25 to 30%, 
and in special cases approximately 60 to 80%. Practical tests have shown 
that such storage containers have a considerably longer service life than 
metal storage tanks and sometimes can be used for up to 10 years without 
corrosion. This is all the more unexpected since it is known per se that 
polyphenylene oxide (PPO) has only a very low water absorption capacity, 
yet at higher temperatures an oxidative destruction is nevertheless 
ascertainable. Despite the elevated temperature typical in the tanks, 
however, it has been found that PPO in the modified form has excellent 
resistance to oxidation and hydrolysis when used as an internal liner. 
In long-term tests of the above-described plastic containers used as tanks, 
however, it has been found that unexpectedly, embrittlement of the PPO 
inner liner was ascertainable after relatively long use, which if the 
container is stressed by jarring or vibration, as can for instance happen 
even from traffic on busy streets, can lead to hairline cracks and hence 
to leaking of the container. Only with extensive tests was it possible to 
ascertain that the cause is the styrene content that the customary 
polyester reaction resins available in commerce have as a rule. The 
unpolymerized styrene in the outer laminate layer attacks the PPO liner 
and hence causes an impairment of its mechanical properties. 
It is therefore the object of the invention to develop plastic containers 
of the above-described type that do not have the demonstrated 
disadvantages and which are suitable as tanks, particularly as hot water 
heaters and also as unpressurized containers, particularly the so-called 
expansion tanks for hot water heating systems.

DETAILED DESCRIPTION OF THE INVENTION 
The manufacture of the plastic containers according to the invention is 
effected in that the liner of modified PPO, which is for instance 
available in commerce from General Electric under the trade name "Noryl", 
is produced in a manner known per se by extrusion blow molding. After the 
flanges, connections and so forth are in place, which need not be glued or 
welded, the liner is used as a core in the winding process. Rotationally 
symmetrical shaped bodies such as the containers in question here, as well 
as shaped bodies of more-complicated design, are wound in a known manner 
with planetary winding machines having three-dimensional numerically 
controlled filament winding. The essential feature is that the glass 
fibers are laid on approximately geodesic lines, so as to attain as 
uniform as possible a tensile and bending strength at every point of the 
container. Geodesic lines are lines of vanishing curvature over a surface 
in the three-dimensional euclidic space. There are surface curves c(t)=[fo 
u](t) of a surface f: U-R.sup.3 (U.ltoreq.R.sup.2, u(t)=(u.sup.1 (t), 
u.sup.2 (t)), which are characterized by the following properties: C(t) is 
a curve of vanishing geodesic curvature. U(t) satisfies the differential 
equations for geodesic lines: 
EQU U.sup.k (t)+.GAMMA.u.sup.i (t) rij.sup.k ou(t)=0, 
where 
##EQU1## 
and rij.sup.k are the Christoffel symbols of the second kind, in classical 
area theory. 
The location of the geodesic line is naturally oriented to the shape of the 
particular body and must be suitably calculated and programmed. For better 
fixation of the fibers, a cross-wound winding can be provided over the 
liner or as a cover of the glass fiber layer. The thickness of the 
thus-wound glass fiber jacket depends on the expected pressure load; with 
unpressurized containers, a single-layer glass fiber layer suffices as a 
rule, while for pressure tanks, depending on requirements prescribed by 
law, a thickness of the glass fiber layer must be planned for that has 
compressive and bending strengths approximately 2 to 10 times as high as 
the maximum pressure to be expected. For hot-water tanks, as a rule, a 
margin of safety of five times the maximal pressure is assumed. 
The final fixation of the glass fiber jacket can be accomplished in two 
ways; for instance, the fiber strands can be saturated with resin or 
thermoplastic in a continuous bath in the winding process and then wound 
under tension onto the rotating shaped body. This can always be done for 
instance whenever reaction resins that do not contain any styrene in the 
so-called premixes are used. If the fixation is to be done with 
unsaturated polyester resins, then first a layer of glass fibers without 
resin additive is wound on, and only with the second layer is a saturation 
of the fiber strands with resin performed. In this way, the monomeric 
styrene present in the premixes is kept from being able to come into 
contact with the PPO or PP liner where it could cause embrittlement. 
Reaction resins that can be used, besides the unsaturated polyesters, 
include epoxide resins, reactive polymetlylmethacrylate (PMMA) resins or 
isocyanate resins. Processing is done in a manner known per se, with the 
addition of catalysts and accelerators. If desired, the outer fiber layer 
can also be saturated with reaction resins suitable for the production of 
expanded plastic, so that an expansion of these resins can subsequently be 
performed by irradiation or temperature elevation. The outer layer of the 
fiber jacket, which serves to fix the fibers, may for instance comprise a 
modified PPO foam, if for particular reasons an increased stability to 
thermal and hydrolytic strains is for instance desired. However, fixation 
with thermoplastics is also possible, either by saturating fibers with 
heated thermoplastics in the bath, or by applying these thermoplastics 
subsequently to the fiber layer. 
The plastic containers according to the invention may be used as tanks for 
aqueous or non-aqueous liquids or for gases, especially in this case for 
moist gases, because they are particularly suitable as pressure tanks and 
with a suitable design of the glass fiber jacket can withstand pressures 
of up to 100 bar. Because of the chemical inertness and the thermal and 
hydrolytic stability, the containers are above all usable for applications 
in which long-term thermal strain under pressure from aqueous solutions is 
to be expected, an example being water tanks and in particular hot-water 
heaters. Tests performed thus far have clearly shown that because of the 
specialized design of the composite material any embrittlement of the 
liner is avoided, and in this way water tanks can be produced that 
withstand a continuous test load that is equivalent to a normal load of 
well over ten years in domestic or industrial use. 
These tests have also found, in particular, if the containers are used as 
pressure tanks for moist gases, that the liner or the inner layer in some 
cases should preferably not comprise PPO or PP as a single-layer system, 
but rather that under certain conditions, multilayer systems that do 
contain PPO and other thermoplastics can preferably be used. 10 Without 
changing the pressure and strain behavior otherwise, it is possible in 
special cases, for instance with moist gases, for the PPO liner to be 
replaced with a liner of other thermoplastics having better resistance to 
aggressive moist gases; suitable plastics for the inner layer may 
optionally be PTFE, i.e., polytetrafluoroethylene; PCTFE, i.e., 
polychlorotrifluoroethylene; PSU, i.e., polysulfones; or PI, i.e., 
polyimides. Preferably, however, in that case, these plastics are used 
with PPO as multilayer systems. An important factor in this connection as 
well, however, is the fact that, as described above, the inner layer or in 
other words the liner is seamless; that is, it has neither a weld seam nor 
a pinched seam at which an attack of aggressive liquids could occur. 
It has also been found that with pressure tanks subjected to severely 
fluctuating pressures it may be advantageous for a kind of buffer layer 
comprising a matrix of reaction resins or thermoplastics not containing 
monomeric styrene, but with a glass fiber content in the range from 
approximately 20 to 40% by weight in the form of short fibers (relative to 
the length of the glass fibers in the outer layer) to be laid between the 
liner and the glass fiber layer comprising long fibers. The term 
styrene-free is understood in this connection to mean a thermoplastic or a 
reaction resin that after curing contains only such amounts of monomeric 
styrene as technically cannot readily be removed. Grades of foodstuffs 
that depending on the copolymers may still have a content of monomeric 
styrene and other volatile compounds up to a maximum of 0.5 to 0.6% are 
considered to be styrene-free in the technical sense. 
This "buffer layer" absorbs extreme pressure strains that for instance with 
flanges of metal could cause the metal edges, with highly fluctuating and 
sometimes very high pressures, to cut into the glass fiber layer in such a 
way that some of the glass fibers would be destroyed and thus the overall 
strength of the container could be reduced. The relatively soft buffer 
layer comprising thermoplastics or reaction resins and a relatively low 
proportion of short fibers absorbs such extreme strains. 
If the containers according to the invention are used as unpressurized 
containers, then naturally such a buffer layer need not be present. It has 
also been found that with unpressurized containers, which after all are 
subjected to a lesser strain, the geodesy of the glass fiber layer need 
not be observed so strictly as with pressure tanks, so that in those cases 
winding of the glass fiber layer can be done in such a way that a greater 
deviation from the mathematically correct geodesy is possible than in the 
case of pressure tanks. However, it has proved advantageous in that case 
as well to apply a cross-wound winding on the liner or as an outer 
covering of the jacket. 
DESCRIPTION OF THE DRAWINGS 
The invention will now be described in further detail in conjunction with 
the drawings: 
FIG. 1 shows a hot-water tank according to the invention in a plan view. 
FIG. 2 is a schematic cross section through the tank wall in a preferred 
embodiment. 
The tank 1 comprises the liner which may have a plurality of layers, not 
visible, as well as a glass fiber layer 3, which is provided on its 
surface with a layer of reaction resin. There may be a layer (not visible) 
intermediate the liner and the fiber glass layer, of thermoplastics or 
reaction resins containing short glass fibers. The container is equipped 
with flanges 2a, 2b at both ends, which are not secured separately but 
instead are joined to the line by the winding of the glass fibers. The 
winding is effected on approximately geodesic lines, which also produce 
the enlargements 4a and 4b at the shoulders of the container. This winding 
is adjoined by a cross-wound winding that can be seen from the outer 
surface. 
In a preferred embodiment, the wall of the container comprises a PPO or PP 
inner layer or liner, which is followed by a glass fiber layer, the outer 
covering of which is in turn formed by a layer of reaction resin. 
EXAMPLE 
In a hot-water heater with a capacity of 100 liters, a suitable seamless 
liner of modified PPO having a wall thickness of 2 mm is produced by 
extrusion blow molding. Preferably, the method is performed such that the 
flanged rings and the closed form are extruded through and then the hose 
is pinched off outside the die. As a result, undercuts can simultaneously 
be produced, which encompass the flanged extruded segment and, when the 
0-ring is introduced, make it impossible for the liner to be forced out of 
the ring. Next, in a manner known per se, a single layer is applied as a 
cross-wound winding and the further layers of the glass fibers are applied 
on approximately geodesic lines, using a planetary winding machine with 
three-dimensional numerically controlled filament winding. Part of this 
fiber jacket is provided as a buffer layer and thus has a thickness of 
approximately 0.4 mm. As the application of the glass fibers continues, 
fibers saturated with reaction resin such as polyester resin are then 
used, up to a layer thickness of a further 0.6 mm, so that the fiber 
jacket, including the outer polyester layer, has a total thickness of 
approximately 1 mm. 
This hot-water heater is designed for a pressure over long-term use of 10 
bar and a test pressure of 30 bar. The tests showed that the bursting 
pressure was approximately 124 bar.