Method of producing a barrier in a thermally insulated container

A method for producing a barrier in a thermally insulated container lined with polyurethane foam for storage or transport of liquefied gases, wherein a plurality of layers of an epoxy resin formulation and a glass-fiber material are applied in a particular sequence resulting in a barrier of superior quality essentially free of pinhole flaws.

BACKGROUND OF THE INVENTION 
The present invention relates to the manufacture of thermally insulated 
containers for storage or transport of liquefied gases. 
Dutch patent application No. 7,400,276, published Mar. 25, 1974, and 
assigned to the assignee of the present invention, discloses a thermally 
insulated container for storage or transport of liquefied gases, in 
particular methane or natural gas, comprising a lining of polyurethane 
foam arranged on the inner surface of a rigid outer shell, wherein the 
lining of polyurethane foam is provided with a barrier consisting of 
glass-fiber material and an epoxy-resin system. Furthermore, said 
published Netherlands patent application discloses a method of producing 
said barrier in a container as mentioned above, which method comprises 
applying an epoxy resin formulation (which formulation comprises an epoxy 
resin and a curing agent) on polyurethane foam already applied onto the 
inner surface of the rigid outer shell, applying a layer of glass-fiber 
material to the said epoxy resin formulation, passing a roller over the 
glass-fiber material and the said epoxy resin formulation to obtain good 
wetting of the glass-fiber material by the said epoxy resin formulation 
and to ensure compaction and allowing the epoxy resin to cure. 
The present invention particularly relates to an improved method of 
producing said barrier, so that the risk of pinhole flaws occurring in the 
barrier is reduced as much as possible. 
SUMMARY OF THE INVENTION 
The method according to the invention comprises the steps of: 
Cutting or grinding the inner surface of polyurethane foam already applied 
onto the inner surface of a rigid outer shell so as to obtain a 
substantially flat cut or ground surface of polyurethane foam; 
Applying a first layer of an epoxy resin formulation, comprising an epoxy 
resin and a curing agent, onto the cut or ground surface; 
Allowing the epoxy-resin formulation of the first layer to gel; 
Applying a second layer of an epoxy resin formulation, comprising an epoxy 
resin and a curing agent, onto the gelled first layer; 
Applying a layer of glass-fiber material onto the second layer when the 
epoxy resin formulation of the second layer is still wet and compacting 
the glass-fiber material, for example, by passing a roller over it; 
Applying a third layer of an epoxy resin formulation, comprising an epoxy 
resin and a curing agent, onto the second layer and glass-fiber material; 
and 
allowing the epoxy resin formulation of the second and third layers to gel.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In the specification and claims pertaining to the present patent 
application, allowing the epoxy resin formulation to gel means allowing 
the epoxy resin formulation to cure partly or fully. 
Referring to the drawings, in FIG. 1 the marine tanker T comprises an outer 
hull 12, an inner hull 1 and a ballast space 13 between the said inner and 
outer hulls. The tanker T is provided with a main deck 14 and a trunk deck 
15. The inner hull 1 functions as a rigid outer shell of the container for 
liquefied gas. The rigid outer shell 1 is normally made of steel, for 
example, grade E steel. A lining 2 of polyurethane foam is arranged on the 
inner surface 3 of the rigid outer shell 1. The polyurethane foam is 
preferably rigid polyurethane foam having preferably closed cells. The 
lining 2 is preferably applied by a spraying method, for example, by means 
of the apparatus as described in applicant's British Pat. No. 1,300,352, 
published on 20th Dec., 1972. The inner surface 4 of the polyurethane foam 
2 is adapted to be in direct contact with the cargo of liquefied gas 16, 
for example, liquefied natural gas at a pressure of about 1 kg/cm.sup.2 
and at a temperature of about -160.degree. C. Above the liquid surface 17 
a vapor space 18 is present. 
Within the lining 2 of polyurethane foam at least one barrier 5 is arranged 
(FIG. 2). If desired, two, three or more spaced apart barriers 5 may be 
present in the lining 2. 
The barrier 5 is produced as follows: 
Assume that polyurethane foam 2 has been applied onto the inner surface 3 
of the rigid outer shell 1 until a certain thickness has been reached, 
depending on the desired location of the barrier 5, for example, at a 
distance of 200 to 250 mm from the inner surface 3. The inner surface of 
the polyurethane foam is cut or ground until a substantially flat surface 
19 is obtained. A first thin layer 20 of an epoxy resin formulation which 
comprises an epoxy resin and a curing agent is applied, preferably by 
spraying, onto the cut or ground surface 19, whereafter the epoxy resin 
formulation of the first layer 20 is allowed to gel. Then a second layer 
21 of an epoxy resin formulation, which formulation comprises an epoxy 
resin and a curing agent, is applied onto the gelled first layer 20. 
Thereafter, a layer of glass-fiber material 22, for example, glass-fiber 
cloth, is applied onto the said second layer 21, when the epoxy resin 
formulation of said second layer 21 is still wet and the glass-fiber 
material 22 is compacted, for example, by means of a roller. The 
compacting of the glass-fiber material 22 ensures good wetting of the 
glass-fiber material 22 by the epoxy-resin formulation of the second layer 
21. After the compacting operation, a third layer 23 of an epoxy resin 
formulation, which comprises an epoxy resin and a curing agent, is applied 
onto the second layer 21, while it is still wet, and further compacting 
carried out as necessary. Thereafter, the said second layer 21 and third 
layer 23 are allowed to gel. In the above manner, a barrier 5 is obtained 
comprising only one layer of glass-fiber material 22 which is shown in 
FIG. 3. In the embodiment of FIG. 3, polyurethane foam 2a is applied on 
top of the third layer 23. If more than one barrier 5 is required, the 
above process is repeated one or more times so that a plurality of 
spaced-apart barriers 5 will be incorporated in the polyurethane foam 2. 
Often, it is desired to produce a barrier 5' as shown in FIG. 4, comprising 
two layers of glass-fiber material. Such a barrier 5' is produced by 
applying a fourth layer 24 of an epoxy resin formulation on the gelled 
third layer 23. This fourth layer 24 comprises an epoxy resin and a curing 
agent. Then a layer of glass-fiber material 25, for example, glass-fiber 
cloth, is applied onto the said fourth layer 24, when the epoxy resin 
formulation of said fourth layer 24 is still wet and the glass-fiber 
material 25 is compacted, for example, by means of a roller. The 
compacting of the glass-fiber material 25 ensures good wetting of the 
glass-fiber material 25 by the epoxy resin formulation of the fourth layer 
24. After the compacting of the glass-fiber material 25, a fifth layer 26 
of an epoxy resin formulation, which comprises an epoxy resin and a curing 
agent, is applied onto the fourth layer 26, while it is still wet and 
further compacting carried out as necessary, whereafter the fourth layer 
24 and fifth layer 26 are allowed to gel. If more than two layers of 
glass-fiber material are required in the barrier 5, the above process is 
repeated one or more times depending on the desired number of layers of 
glass-fiber material. In the embodiment of FIG. 4, polyurethane foam 2a is 
applied on top of the cured fifth layer 26. 
Preferably, the epoxy resin formulation as mentioned in the above, includes 
a thixotroping agent, and/or, if necessary, a flexibilizing component, 
such as a flexibilizing epoxy resin. Such epoxy resin formulations and 
respective components thereof are well-known in the art and are readily 
available commercially. 
For further details about a preferred epoxy resin formulation, reference is 
made to the above-mentioned published Netherlands patent application. 
A problem with barriers in heat-insulated containers for liquefied gases is 
that sometimes pinhole flaws occur in these barriers. These pinhole flaws 
can be detected in a simple manner by applying onto the cut or ground 
polyurethane foam surface a fluorescent dye before applying the barrier. 
If pinhole flaws are present, irradiating the surface of the barrier with 
ultraviolet light will cause fluorescence at the locations of the pinhole 
flaws. Pinhole flaws in the barrier have the disadvantage that some 
liquefied gas may penetrate through the barrier and may collect behind the 
barrier. When the container is emptied and the temperature of the 
container raises to ambient temperature, the said collected liquefied gas 
will evaporate and expand and may cause damage to the barrier. 
The main object of the method according to the invention is to produce a 
barrier of very good quality so that the chance that pinhole flaws are 
present in the barrier is very small. 
If desired, the barrier need not to be incorporated in the lining of 
polyurethane foam. Instead, it is possible to arrange the barrier on the 
inner surface of the polyurethane foam so that, when the container is in 
use, the liquefied gas stored in the container is in direct contact with 
the barrier and is not in direct contact with the polyurethane foam. Said 
barrier can be produced as well by means of the method according to the 
present invention. 
The quantities of the epoxy resin formulation applied on top of the gelled 
first layer 20 range from 80-90% of the total quantity for the second 
layer 21 and from 10-20% of the total quantity for the third layer 23. 
Similarly, the quantities of the epoxy resin formulation applied on top of 
the gelled third layer 23 range from 80-90% of the total quantity for the 
fourth layer 24 and from 10-20% of the total quantity for the fifth layer 
26.