Pipes and tubular conduits – Distinct layers – Bonded to each other
Reexamination Certificate
1998-09-04
2001-01-23
Hook, James (Department: 3752)
Pipes and tubular conduits
Distinct layers
Bonded to each other
C138S149000, C138S153000
Reexamination Certificate
active
06176269
ABSTRACT:
FIELD OF INVENTION
The invention relates to co-extruded multilayer plastic pipes comprising one or more inner pipes surrounded by a middle layer of a softer material, and an outer pipe enclosing said middle layer surrounding said one or more inner pipes. Preferably the multilayer plastic pipe of the invention comprises one inner pipe.
BACKGROUND OF INVENTION
Plastic pipes of the type described above, comprising one thin-walled inner pipe, are previously known. They are used, for example, as underground drain pipes, pressure pipes and cable ducts. They are more complicated to manufacture than conventional single-layer pipes, but since the consumption of material and thus also the weight of the pipe are lower than with single-layer pipes having corresponding properties, multilayer pipes are somewhat less expensive than conventional pipes. Nevertheless, their use is very limited particularly in northern latitudes. The main reason for this is that pipes of this kind have poor mechanical properties as compared with conventional single-layer pipes.
A conventional three-layer underground drain pipe has the following construction, which complies with the standards published in the field (the outside diameter of the pipe being 315 mm):
inner layer of hard PVC plastic, thickness about 1.4 mm, elasticity modulus about 2000 MPa, density about 1400 kg/M
3
,
middle layer of foamed PVC plastic, thickness about 9.4 mm, elasticity modulus about 800 MPa, foam density about 800 kg/m
3
,
outer layer of hard PVC plastic, thickness about 1.4 mm, elasticity modulus about 2000 MPa.
The ring stiffness of such a pipe is about 8.8 kN/m
2
, which is sufficient for underground laying. The ring stiffnesses of the different pipe layers are as follows: the inner pipe (1) about 0.0167 kN/m
2
, the middle layer (2) about 1.8 kN/m
2
, and the outer pipe (3) alone about 0.0136 kN/m
2
.
In the order of magnitude, the ring stiffnesses are 2, 1 and 3. This is the typical and predominant construction of foam pipes available on the market. Despite the foaming and thus the lowest elasticity modulus, the middle layer is the stiffest and the most load-bearing structure. The inner pipe is typically the second stiffest structure.
If the weights of the different layers of three-layer foam pipes available on the market are examined, another predominant dimensioning principle can be seen: the proportion of the weight of the solid layers to the total weight of the pipe is always less than 45%. In the example described above, the weight of the inner pipe was about 1.9 kg/m, of the middle layer about 7.4 kg/m, and of the outer pipe about 2 kg/m. The weight proportion of the inner pipe and the outer pipe taken together to the total weight of 11.2 kg was thus 34%.
U.S. Pat. No. 4,364,882 discloses a conventional PVC foam pipe. The PVC is foamed to a density of 500 kg/m
3
, which in fact is the lowest value obtainable by conventional techniques. The typical foaming degree of PVC foam is 57%, whereby the density is 800 kg/m
3
; if the density is lower than this, the strength properties of the PVC foam are weakened. The patent discloses a pipe having an outer diameter of 315 mm and the following structure: the thickness of the inner pipe 1.25 mm, the thickness of the middle layer 9 mm, and the thickness of the outer pipe 1.25 mm. The total thickness of the pipe is thus 11.5 mm, and the total weight is 7.63 kg/m; thus the pipe disclosed is 29% lighter in weight than a conventional pipe with a corresponding stiffness.
Further the following characteristics of the pipe above can be calculated: the weight of the inner pipe 1.61 kg/m and the ring stiffness 0.013 kN/m
2
; the weight of the middle layer 4.32 kg/m and the ring stiffness 1.41 kN/m
2
; the weight of the outer pipe 1.72 kg/m and the ring stiffness 0.011 kN/m
2
. It can be seen that, as compared with the outer and inner pipes, the ring stiffness of the foamed middle layer is more than hundredfold, and that the weight proportion of the outer and inner pipes taken together is 44%.
The example described above shows that the use of foamed plastic has the advantage that it saves considerable amounts of material (i.e. cost saving) and the pipe construction becomes lighter. Such a use of material of “poorer quality” in the middle layer is appropriate in this connection, since this layer is the least subjected to mechanical stresses, such as wear and stress strains, and to physical and chemical stresses, such as UV radiation and various impurities.
On the other hand, when the foaming degree of the middle layer is increased, or its density is reduced, the properties of the foamed material are significantly weakened. So far the highest foaming degree used has in practice reduced the density of the material to half of the density of unfoamed material. If a higher foaming degree were used, the strength of the foam would be considerably weakened, and it has been considered impossible to construct a pipe of good quality using such foam. Although in the prior art solutions the foaming, for instance, has been maintained within such a range that the foamed material still has relatively good mechanical strength properties, it has not always been possible to avoid damages extending as far as the inner pipe. Nor has the use of a foamed intermediate layer resulted in quite as significant cost savings as originally intended.
SUMMARY OF INVENTION
The object of the present invention is to provide a co-extruded multi-layer plastic pipe where the inner pipe is protected more efficiently than in prior art solutions and at the same time to reduce the manufacturing costs of the pipe.
A further object of the present invention is to provide a co-extruded multilayer plastic pipe which has better mechanical properties, e,g. impact strength and ring stiffness, than those of the corresponding known pipes and which is lighter in weight in comparison with single-layer pipes and the corresponding known multilayer pipes.
Still another object of the invention is a method for producing a co-extruded multilayer plastic pipe comprising one or more inner pipes surrounded by a middle layer which is made of a softer material, and has a lower density than the outer and inner pipe, and an outer pipe enclosing said middle layer surrounding said one or more inner pipes.
A further object of the present invention is to provide a method for producing a co-extruded multilayer plastic pipe, said method enabling the production of pipes in which the middle layer of foamed plastic has a very low density and in said method foaming being carried out as in situ foaming using compressed gas.
A further object object of the present invention is to provide an apparatus for producing co-extruded multilayer pipes according to the invention.
The multilayer pipe according to the present invention has the essential feature that the most important portion, i.e. the inner pipe, is not broken or deformed under stresses of various kinds.
The above described properties can be achieved now with a multilayer plastic pipe of the present invention where the weight proportion of the polymer in the middle layer to the total weight of the plastic pipe is up to 55%.
According to the invention the most important portion of the construction, i.e. the inner pipe, can be protected by softer outer layers, i.e. layers which are more easily deformed, whereby the adhesive forces between the interfaces of all the layers are as small as possible and adjustable. The inner pipe thus remains circular and undamaged even if the outer pipe becomes oval as a result of compression or even breaks as a result of impact stress caused by a sharp-edged object, e.g. a stone, when the pipe is laid in an underground excavation.
The above-mentioned adhesive forces have unexpectedly been found to have an extremely great influence on the durability of a structure, particularly in the case of impact stresses. In the prior art three-layer pipes of the type described, the foamed intermediate layers have obviously—despite the foamed structure—been too hard and too tightly attached to the out
Hook James
Ladas and Parry
Uponor Innovation AB
LandOfFree
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