NO163903B - FLEXIBILIZED FOAM PLATE PLATE AND PROCEDURE FOR FLEXIBILIZATION OF SUCH PLATE. - Google Patents

Description

Rigid closed-cell thermofoam plastic has been widely used

as a thermal insulation material because it is light, has good compressive strength and great insulating properties. The stiffness and the fool-

equal elasticity to foam plastic of this type are, however, factors that make it ill-suited for use on curved surfaces, such as pipelines and cylindrical or spherical tanks. To cut to. customized pieces or molding to order entail additional manufacturing problems and costs. If such foam plastic should still be pressed on

space on., a curved surface, the closed cell structure will often crack or break up, so that it loses its insulating ability.

US Patent No. 3,159,700 to Nakamura describes an alter-

native process for directional flexibilization of rigid foam<p>sheets by a partial compression or crimping of an expanded foam sheet in a direction substantially perpendicular to the direction of the desired flexibility. This process is designed to provide ripples in the cell walls of the foam without

break up the foam cells or cause any significant loss of compressive strength in other directions. By repeating the process mainly at right angles to the first direction, a two-dimensional flexibilization is obtained, which gives a foam product which to a limited extent can be adapted to a compound curve.

Such properties are particularly valuable for rigid foam plastic sheets that are used as low-temperature insulation for pipelines, tanks and other large containers for transport and storage.

ring of low temperature fluids. Siike flexibilized pieces or sheets of expanded foam plastic are easy to assemble by spiral winding technique according to US patent no. 3,206,899

to Wright and No. 4,017,346 to Smith.

However, the insulation requirements for the transport and storage of liquid petroleum gas (LPG) and cryogenic fluids, such as liquid nitrogen, require an even higher resistance to water vapor re-penetration, while at the same time the compressive strength must be able to maintain

maintained during application and during use. Cracking and breaks in the cell walls must be reduced to a minimum.

The purpose of the present invention is therefore to provide a synthetic foam resin which

(1) can easily be applied to a curved surface and then heated to maintain the curved shape. (2) has improved bending capacity, cracking, breaking and tearing resistance, (3) effectively maintains the compressive strength over a long period of time and the required insulation properties when storing and transporting liquefied natural gas and cryogenic fluids at low temperature, and which (4) has great resistance to shrinking as well as permanent cracking resistance in biaxial directions, which is essential to be able to withstand heavy loads under cryogenic storage conditions.

It has been discovered that rigid foam sheets with improved elongation against water vapor penetration properties, which are particularly desirable for cryogenic insulation, can be made more pliable by mechanical compression of certain expanded, closed-cell foam types having carefully selected structural and physical properties, including aging or curing after the expansion.

The invention thus relates to flexibilized foam plastic sheets according to claims 1 and 2 as well as a method for flexibilizing foam plastic sheets according to claim 5.

The resulting flexible foam board has improved flexability and bursting characteristics, which are particularly desirable for low temperature insulation. With a mainly closed-cell polystyrene foam having a mass density of 20 - 60 kg/m<3>, a water permeability in the Y-axis direction of less than 1.0 g/m<2>.hour can actually be achieved by the test method according to ASTM C -355, and this is a stable situation that is

effective for long-term insulation of cryogenic storage tanks.

Reference will now be made to the drawings, where fig. IA, IB and 1C and 2A, 2B and 2C are microphotographs (magnification 50x) of one- and two-dimensional flexible foam plastic according to the preferred examples 123 and 223 according to the invention, and show the cell structure in those in fig. 3 showed X, Y and Z axis directions.

As shown in fig. 1 and 2, the flexible foam sheets according to the invention are characterized by an anisotropic cell wall structure, where the ripples in the cell walls are directional. For the one-dimensional flexible plastic foam according to fig. 1 can thus be seen that the ripples in the cell walls in the X-axis direction (fig. IA) are significantly fewer than those seen in the Y- and Z-axis directions (fig. IB and 1C). For the two-dimensional flexibilized plastic foam, the cell walls are generally less rippled in the X-axis direction and Z-axis direction (Figs. 2A and 2C) than in the Y-axis direction (Fig. 2B).

Because of the small dimensions and the polygonal shape of the cells in the foam, it is. difficult to express the exact distribution and location of such ripples in the cell structure. For the sake of simplicity, this distribution shall be seen as a parameter and described with reference to that in fig. 3 showed three-dimensional coordinate system. For a normal sheet of foamed thermoplastic, the coordinates X, Y and Z correspond to the length in the machine or the direction of extrusion, the thickness or the width of the plate.

The anisotropic ripples, in combination with the properties of the resin used to form the cell membranes and the foam plastic density, will be important factors in the flexible foam plastic. Such physical properties as axial fracture elongation and water vapor permeability constitute a fairly accurate indication of the types, location and distribution of the anisotropic ripples.

The present invention is greatly influenced by the properties of the expanded foam plaster sheets or plates.

The synthetic foam plastic that is used must therefore have an essentially closed-cell structure and may include foam plastic that has been expanded by extrusion or that has been cast and expanded on a substrate. However, foam sheets expanded by extrusion with a substantially rigid and closed-cell structure are preferred. Also important are the foam boards' density, cell size, compressive strength and insulating ability, which in turn depends on the synthetic resin polymer used to produce the original foam plastic.

Suitable synthetic resins are mainly composed of styrene, vinyl chloride, vinylidene chloride, methyl methacrylate or nylon, including copolymers of these and physical mixtures of these resins. According to the present invention, resins are preferred which contain a main component of styrene or a styrene monomer, such as «--methylstyrene and o-, m-, p-vinyltoluene and chlorostyrene. It is also possible to use copolymers of styrene or styrene monomers and other monomers that can be copolymerized with these, such as acrylonitrile, methacrylonitrile, methyl methacrylate, maleic anhydride, acrylamide, vinylpyridine, acrylic acid and renacrylic acid. . However, it is most preferred according to the present invention to use polystyrene resins which mainly consist of polymerized styrene and preferably polystyrene resins see: 1 contain 0.3% by weight or less of styrene monomer as residue cg 0.5 to 1.5 weight in styrene oligomers, primarily 'dimer cg trimer. Polystyrene resins containing such amounts of styrene monomer and styrene trimer form, in the expanded state, a foam that has a very evenly distributed density and cell size as well as improved resistance to repeated compressions. Foams of such polystyrene resins are very well suited for flexibilization in one or two directions.

To improve toughness, rubber can be mixed with such monomers before polymerization, or added to the system after polymerization. The aforementioned resins can also be mixed

with other polymers, as long as the desired properties of the styrene resins are not adversely affected.

Choice of foam boards

In order to achieve the desired flexibilization and the properties that are essential for low-temperature insulation, a careful selection of several properties is required before the flexibilization. It has thus been found to be of great importance for the present invention that the synthetic foam plastic has (1) a mass density of approximately 20 - 100 kg/m<3>, preferably about 20 - 60 kg/m<3> for flexibilization in one direction, (2) a cell size in the Y direction of about 0.05 to 1.0 mm and (3) a compressive strength in the Y direction of at least 1.8 kg/cm^.

When investigating the interaction between the foam density (kg/m<3>) and the cell size (mm), especially the cell size y in the Y direction, a group of flexible foam sheets was developed with a range of densities and cell dimensions in the Y direction, with the compressive strength in the Y- the direction as a parameter for compression resistance, the tensile strength in the X-axis and Z-axis directions as parameters for breaking and tearing resistance for the foam types used, as variations in the X-axis and Z-axis directions constitute parameters for uniformity during manufacture or qualities.--as well as the thermal conductivity in the Y direction.

Typical results are given later in Tables 1 and 2, which are based on a total assembly from a series of samples, and suggest that the foam boards according to the present invention must have a mass density of approximately 20 - 100 kg/m<3>, an average cell dimension jen y of 0.05 to 1.0 mm and average cell size ratios y/x and y/z ~ 1.05" The foam sheets should preferably be made up mainly of cells where the largest axis is oriented along the Y axis, and where the average axial size ratio y/x and y/z is ].10

to 4.0. If the average axial size ratio p/x and y/z exceeds 4, the balance between dimensional stability,

the linear expansion coefficient and the tensile strength are lost.

F lexibility by compression

Synthetic foam boards that have the required mass density and anisotropic cell structure and cell size can be flexed by compression in one or two axial directions, as described in US patent no. 3,159,700 to Nakamura, thereby producing the good water vapor barrier and other desired properties for low temperature - or cryogenic insulation. However, it is with carefully managed conditions.

Figs 4 and 5 show schematic sketches of the equipment for the compression during the flexibilisation. In the equipment in fig. 4, infeed rollers 1 and 2 and outfeed rollers 3 and 4 are arranged which are at a suitable distance from each other. The flexibility equipment which is shown in fig. 5 is equipped with infeed belts 9 and 10 and outfeed belts 11 and 12 which are also located at a suitable distance from each other. These roller pairs or belt pairs fit tightly against the expanded foam board. Reference numbers 5 and 6 in fig. 4 and reference numbers 13 and 14 in fig. 5 suggests installation pressure means which should be able to be precisely controlled, because otherwise the foam board will experience significant compression in the thickness direction if the pressure becomes too great.

During operation, the infeed rollers or belts are driven somewhat faster than the other pair (the outfeed pair), so that the foam board is pressed together in the longitudinal direction between the infeed and outfeed rollers or belts. According to the present invention, the foam board is first compressed in the longitudinal (X-axis) direction. Then, if desired, the flexible board in one direction can be subjected to a compression in another direction at right angles to the longitudinal direction, namely in the side (Z-axis) direction, thereby providing a plate that can assume a compound curvature.

As previously stated, the flexibilization conditions must be selected and controlled precisely.

Particularly important are:

(a) selection of expanded foam boards that have uniform quality throughout the board,

(b) minimal aging of the foam board after expansion,

(c) short compression zone, and

(d) stepwise compression for high elongation flexible foam sheets.

A uniform quality of the initially expanded foam board is necessary because the foam plastic is mechanically compressed - during the flexibilization - in only one direction, axis by axis at a point in time, e.g. first in the X direction and then in the Z direction, while being held in the Y direction. It is therefore necessary that the foam plastic has a minimal variation in its mechanical properties throughout the foam board, especially when it comes to compressive strength.

The significance of the foam being aged or cured after the extrusion or expansion and before the flexibilization is shown in fig. 7 and 8. Example 3 shows additional foam-free samples that have been aged for varying periods of time before flexibilization in the device according to fig. 5, and the foam plastic samples were assessed with regard to water vapor penetration and elongation, properties that are of particular importance for foam plastic s.oin is used as. low temperature insulation or cryogenic insulation. These results suggest that the foam should be flexibilized in the freshly extruded state, ie within 10 days (240 hours) and preferably within 3 days (72 hours) or less. It can be advantageous to carry out the flexibilization in a work line, shortly after the extrusion of the foam board, e.g. after 6 minutes, so that the plastic cools down.

Under control of the compression or compress jbns-bet:inuels, the flexibility of foam board thicknesses from 10 to 300 mm has been made without significant loss of compressive strength in the Y-axis direction, water vapor barrier properties and other wishes. properties. For plates that are thicker than approximately 35 mm, the flexibilizing device according to fig. 5. The extension of the foam sheets treated in this flexibilizing device can be controlled by regulating the distance between the infeed and outfeed belts. The best results are obtained if the compression length D is a maximum of about 300 mm, preferably 200 mm or less, where the duration of the compression is at least one second. A processing line speed of 5 to 40 m/min can be achieved with good results.

For thicker insulation, flexible boards can be laminated in the desired shape, using small amounts of a binder which <p>is applied sporadically to reduce the effect of the binder on the properties of the laminated foam boards.

Flexible foam boards for low temperature insulation

This flexibility of foam<p>lates is mainly achieved by a controlled design of anisotropically oriented ripples in the foam cell walls in such a way that the entirety of the foam is not weakened unnecessarily or that the cell walls do not crack open with the result that there is poorer insulation ability and resistance to water vapor penetration . As the foam cells are very small and have a polygonal shape, it is very difficult to define the exact location of such ripples in relation to the cell shape and cell structure. However, the water vapor permeability in the flexibilized foam sheets indicates a certain stretching or breakage in the Y direction for the cell walls. The extension<p>rose at breaks in the three axial directions is also a measurable parameter for the extent, location and distribution of the ripples. Typical results are the given examples and especially in tables 3 and 4.

It is clear from tables 3 and 4 that the foam boards which have been treated according to the present invention must have a water vapor permeability Py which is equal to or less than 1.5 a/m 2.hour in order to prevent or reduce the destruction of the heat-insulating properties over a long period of use. The water vapor permeability should preferably be 1.0 g/m^.hour or less.

In addition to the water vapor permeability in the Y direction for de-flexible foam sheets, the elongation at break in the three axial directions are useful parameters for the extent, location and distribution of the ripples and for the serviceability of foam sheets under such harsh conditions as may occur in 'storage tanks for liquid nitrogen. An assessment of variations in the elongations at break in the X and Z directions shows the uniformity of the ripple extent through the foam board, while changes in the Trar conductivity in the Y direction over time reflect loss of thermal insulation due to moisture absorption after: prolonged use under loads - i The Y direction. Cryogenic tests at approximately -160°C and -196°C also show the crack resistance in foamolates used as thermal insulation in tanks for liquid natural ash or liquid nitrogen.

The preferred polystyrene foam sheets exhibit excellent cryogenic insulation properties without imposed reinforcement. The flexibility and heat formability of such boards is particularly beneficial in the insulation workplace. To reduce multiaxial stresses in the foam sheets after application or to improve the thermal properties, two or more foam sheets can be bonded together, so that thick foam sheets are formed that are biaxial

stretch marks. These can also be coated with metal foils or they can be combined with synthetic plastic films that have greater resistance as gas barriers.

The present invention also provides improved synthetic foam sheets that can be placed on small diameter pipes by adjusting the stretchability of the foam in the bending direction in accordance with the outer pipe diameter and foam thickness. Other tests with pipes with an outer diameter of 114 mm confirmed the applicability of one- or two-dimensional flexible foam sheets to several differently curved surfaces, including pipes and cylindrical and spherical tanks with different curvatures.

Such samples are representative of the flexibility and of the applicability on curved surfaces, of the cryogenic insulation properties and internal characteristics required in the practical use of such foam boards. The flexible foam sheets according to the present invention are in reality very significant improvements of the previously known foam products. They have become increasingly important as thermal insulation for the storage and transport of LNG, for cooling Food storage <p>g for external walls in buildings. These foam boards provide an effective thermal insulation that can easily be fitted to such constructions at the workplace.

The present invention shall be illustrated in the following with the following preferred test examples, where the processes and test examples to be described later are used. Unless otherwise stated, all parts or percentages will be based on weight.

Polish resins

The polystyrene resins used in the extruded foam sheets were selected from common stock after analyzes of volatile residues (primarily styrene and ethylbenzene) and oligomers (styrene dimers and trimers) by gas chromatography and use of a flame ionization detector. In the case of oligomers, where the resin is dissolved in methyl ethyl ketone, the polymer is precipitated with methanol, and the supernatant liquid is analyzed. These resins have an intrinsic viscosity of about 0.83 measured in a two-hat solution at 30°C.

Extruded foam boards

The polymers were expanded into a rigid, substantially closed-cell plastic sheet in an extrusion/foaming system. consisting of a screw extruder, mixing feeder for foaming agent, a cooler and a plate die. A mechanical mixture

of 100 parts >polystyrene resin, 2 parts fire retardant and 0.03 to 0.1 part nucleating agent was more particularly fed continuously to the extruder along with 12 to 17 parts of a 50/50 mixture of dichlorodifluoromethane/methyl chloride as a blowing agent. The thermoplastic mixture is kneaded under pressure, cooled to an extrusion temperature of about 90 to 118°C, extruded through a die and expanded into foam. The extrusion conditions were controlled so that the foam sheet had a cross-section of 110 mm x 350 mm, and the axial cell dimension ratios y/x and y/z were approximately 1.1 to 1.25 resp. 1.1 to 1.17. The cell dimension in the Y direction and the mass density D were varied in the range 0.07 to 1.6 mm resp. about 21.5 to 77 kg/m<3>. Styrofoam lighter than 21 kg/m^ was subjected to a secondary expansion by treating it with steam at 100°C for two to six minutes. The resulting foam plastic had a mass density of from 15.5 to 20 kg/m 3. Analyzes showed that essentially no loss of volatile substances or oligomers occurred during the extrusion process.

Directional flexibility

The skin-like coating was removed from the net extruded foam to provide foam sheets having a cross-section of approximately 100 mm x 300 mm and a length of 2000 to 4000 mm. These foam sheets were made flexible by mechanical compression in the X-axis direction and then in the Z-axis direction to obtain flexibility in two directions using that in fig. 5 displayed equipment. Characterizing conditions during compression were:.

Trial procedures

The resulting flexibilized foam sheets were then subjected to standard testing procedures. The individual test results are expressed or classified on a general scale such as: • Good (GO) - foam quality as desired or assumed

Passable (PA) - foam quality of a commonly known type.

Unacceptable (UN) - foam quality that cannot be accepted,

and then a total composition assessment was made according to the scale:

Excellent (EX) - classified as good in all tests. Good (GO). - classified as good/passable in all samples. .Passable (PA) - classified as passable in all tests.

Unacceptable (UN) - classified as unacceptable in at least

one of the samples.

(1) Foam density

Standard test pieces, typically a 50 mm cube or a 25 mm x 100 mm x 100 mm plate, were cut from the central portions of foam<p>sheets without a skin coating, and the weight (q) and volume (cm") were determined and the foam density calculated as the average of at least three test pieces.The density variation was calculated using the formula: makes a useful measure of foam uniformity:

(2) average cell size and shape

The average cell dimension x, y and z in the X, Y« and Z directions in the coordinate system of fig. 3 is measured according to the ASTM D-2842 method, where nine test pieces cut in the aforementioned manner are used. The cell shape parameters are then calculated as preferred between the average cell dimension y in the Y direction and the average cell dimensions x and z in the X and Z directions.

The variations in the mean cell dimensions constitute a measure of the uniformity of the foam according to the following rating scale.

(3) Compressive strength

A total of six to twelve 50 mm cubes were cut from each foam board in a standard pattern, and each specimen was subjected to an axial compressive strength test in the unflexified direction in accordance with ASTMD-1621. The average compressive strength is assessed according to the following scale:

(4) Tensile strength and variation

From a foam board without a skin coating, twelve 5 mm sample cubes were cut out in a standard pattern. In accordance with ASTF D-162 3B, each specimen was subjected to a tensile strength test in the X-axis direction using a jig or fixture at each end. The mean value of the measured tensile strength .S-j. to and the tensile strength variation was calculated as follows:

The average tensile strength in the Z direction and the variations in this were measured in a similar way for twelve other <p>robe pieces.

(5) Percentage elongation at break

In accordance with ASTM D-16 2 3B, three groups of 12 test pieces in the form of 50 mm cubes were subjected to tensile strength tests in the X-axis, Y-axis and the Z-axis directions, to determine the elongation at break Gx, Gy and Gz, from which the percent elongation at break Ex, Ey and Ez were calculated using the following formula: Percent elongation at break (Ex,Ey,Ez) =GX^q\ ^ (lT— >' lnC(%> Then the mean percentage elongation Ex, Ey and Ez for the respective test groups and their variations were calculated by the following formulas: 12

(Ex,Ey,E?)

Mean percentage elongation at break Ex,Ey,Éz=——-^

It is also useful to calculate the ratios Ex/Ey and Ez/Ey as a further measure of the foam quality. (6) Thermal conductivity A flexible foam board was cut into test pieces each measuring 200 x 200 x 25 mm. Each sample was then<p>aged in a chamber partially filled with water at 27°C. The specimen was fixed in the chamber approximately 30 mm above the water surface, and a cold plate cooled to 2°C by recycled cooling water was brought into contact with the barrel face of the specimen. After aging for 14 days, the sample was taken out and the surface was lightly dried with gas. the thermal conductivity ^<>><1> of the aged specimen is measured in accordance with ASTM C-518, and the ratio of A.' and the initial thermal conductivity ^> of the test piece pipe aging was calculated.

(7) Water vapor permeability

Three circular test pieces with a diameter of 80 mm and a thickness of approximately 25 mm were cut out of flexible foam board, and the water vapor permeability of the test pieces was measured in accordance with ASTM C-355 and using distilled water. From these measurements, the water vapor permeability was calculated using the following formula:

where G....... change in the weight of the test piece (g)

A....... the area exposed to water vapor transfer

(n\2)

t the time in which the weight of the test piece changes with G

grams (hours)

For low-temperature insulation, it is highly desirable to have a water vapor permeability of less than 1.5, and preferably less than 1.0 g/m . hour.

(8) Cryogenic samples

A. Three 20 mm x 100 mm x 1750 mm test pieces were prepared from flexible foam board and wrapped around a stainless steel tube 36 and the opposing faces (YZ faces) were butt welded together as shown at 40, 41 and 42 in Fig. 10. The pipe test pieces were quickly immersed in a cryostat filled with liquid nitrogen, so that all the test pieces lay well below the liquid surface. After being submerged for 5 hours, they were taken out of the cryostat and left at room temperature for 5 hours'. After four rounds of such treatment, the three test pieces were carefully studied for visual endrins, which may include cracks, tears or breaks.

Good ... No visible cracks or tears

Unacceptable ... cannot be un<p>wound without cracking

B. In another test, a flexible shed test piece with thickness 50 mm, width 170-270 mm and length 300 was smoothed by machining the top and bottom rafts. After marking the X and Z axes on the edges, the top and bottom of each specimen were covered with 12 mm thick veneer (corresponding to Japanese agricultural standard), using a commercial cryogenic ooly-urethane adhesive (Sumitac EA90177, manufactured by Sumitomo Bakelite Ce, Ltd., Japan) to bond the surfaces together. The adhesive was cured by placing the sample panel under a pressure of 0.5 kg/cm2 for 24 hours at 23°C.

1. Cryogenic sample at - 160°C

Each cryogenic sample panel 34 was placed in a liquid nitrogen-cooled cryostat box where the internal temperature is controlled to

-160°C j; 5°C by controlled gasification and distribution of liquid nitrogen. After 5 hours, the sample<p>anel was quickly removed and left at room temperature for approximately 1 hour. This process was repeated

<in times. After the fourth time, the test panel was visually examined for cracks in the four exposed surfaces of the foam test piece. One hour after removing the sample from the cryostat, the veneer cover sheets were then removed with a cutting machine. Then

a 10 mm thick disc of the foam was cut from the top surface, and a mixture of surfactant and dye in water was applied to the surfaces of the cut foam to detect any cracks therein.

2. Cryogenic sample at -196°C

For this sample, a cryogenic box was used which was partially filled with liquid nitrogen. The veneer-coated test panels were immersed in liquid nitrogen and placed on triangular steel supports which were attached to the bottom of the box. A steel plumb bob pre-cooled in liquid nitrogen was placed on top of the test panel, and the panel was kept submerged for 30 minutes. The sample panel was then taken out and left at room temperature for one hour under forced ventilation. After having. After repeating this process four or more times, an examination of surface cracks or internal cracks was now carried out in the same way as described above under test B(l).

(9) Cryogenic pipe insulation

A. Flexibility. Three pieces of flexible foam sheets 200 mm wide, 500 mm long and 25, 37.5 and 75 mm thick were bent to the curvature of a steel pipe 54 with an outer diameter of about 114 mm by applying a bending stress in the Y-axis direction, where Z- the axis was arranged parallel to the axis of the pipe 54, as shown in fig. 11. The test piece was bent until it was in good contact with the outer circumferential surface of the tube over an area exceeding the outer surface area of a semi-cylindrical portion of the tube (the portion above the centerline A-A shown in Fig. 11).

B. Heat formability. The flexible foam pieces were bent to the outside curvature of a steel pipe 54 with an outside diameter of 114 mm, where the Z-axis was arranged along the pipe axis. Marks are put on. the cut edge of the tube 54 on diametrically opposite sides of that in fig. 12 showed center line. A - A. The bent test piece was then completely covered with a galvanized, 0.3 mm thick steel plate 55 and the opposite side edges of the foam core piece were secured with stretch bands 57. The coated test piece was then placed in a hot air oven with the stretch bands 57 down and heated to 85°C for 45 minutes. After being removed from the oven, the specimen was cooled to room temperature for two hours. Then the galvanized cover plate 55 was removed and the gaps 58 and-59 from the outer ends reaching the aforementioned marks to the points of intersection with the center line A-A and the inner wall of the test piece 56 were measured and classified as follows:

C. Thermal insulation test. Flexibilized foam pieces cut to a size of 37.5 mm x 200 mm :■ : 500 mm were warrnformed as before in two layers and then Z-axially cut to form inner and outer semi-cylindrical insulating cups for a pipe with an outer diameter of 114 etc. The sample cup pieces were then fitted onto the 114 mm stainless steel tube which was 800 mm long and had flanges at each end and were attached with a cryogenic polyurethane adhesive. The joints for the outer cover bowls are offset from the joints for the inner cover bowl. The entire pipe section was then coated with a 2.5 mm thick waterproof layer of polyurethane coating. After four days of aging, the tubes were thus coated

connected to a cryogenic sample line and filled with liquid nitrogen. The interior of the stainless steel tube is kept at a temperature of -196°C for 6 hours. Liquid nitrogen was then emptied out and the coated or covered tube was placed at 23°C and 80% re-

relative humidity for 12 hours. The aforementioned test cycle was repeated four times at the same time as the surface conditions were studied on the waterproof layer 66, also including water condensation and icing.

The results were assessed now in the following way:

Immediately after the above tests, the waterproof coating or cover and the foam insulation were carefully removed and visually examined for cracks using farneoo<p>L?;.-ning if necessary.

Example 1 Flexibility in one direction

Commercial polystyrene resin containing 0.20% by weight of volatile residues comprising styrene monomer and 0.87% by weight oligomers comprising styrene trimers (here PS Harpiks A) was used, and a number of foam sheets were prepared for flexibilization in one direction. The extrusion conditions were controlled to yield a foam board having a cross section of about 110 mm V '3.50_ in.m and having a bulk density of about 21.5 to 60 kg/m<3>. Skin layers were removed from each foam board and the resulting foam board was cut into three smaller boards that were square with side 100 mm and 4000 mm long.

Pre-drawn examples 101-112; Reference Examples RlOl-1 07

After one day's aging, the foam sheets were made flexible by compression in the machine direction (X-axis) as the equipment 1 P- Mcc fig. 5 was used together with the typical conditions described for the above procedures. De f lexibi lise? c foam sheets according to the preferred examples 101-112 were evaluated regarding density, cell dimensions in the Y direction, cell shapes as defined by y/x and y/z, compressive strength (in the Y and Z directions) and the tensile strength and elongation in the X -direction at break, and these results are shown in Table 1. In these examples, the axial dimension ratios y/z were of the order of 1.00 to 1.25.

For comparison, other expanded foam sheets of PS Harpiks A, but which lacked the desired foam characteristics, can be specified, and these were flexible in a similar way with the results shown in table 1 as reference examples.

Based on the results shown in table 1, the mass density of the foam was plotted in the diagram in fig. 6A against the mean cell dimension y in the Y direction. The coordinates for the foam test pieces that are not satisfactory for the present invention are marked with x.

As shown in fig. 6A, foam boards according to the present invention must have such a cell dimension y in the Y direction and such mass densities D that the drop in the pentagonal area defined by the five coordinates (1,0, 43)', ( 1 , 0, 20), (0 , 05, 24 ), (0,05, 60) and (0,1, 60), and preferably in the tetragonal area defined by the coordinates (0,8, 42), (0,8, 23), (0, 07, 26) and (0, 07, 57) . The mass density D and the cell dimensions y in the Y direction for these foam types satisfy the following formula: and preferably;

Example 2 Flexibility in two directions.

By using the same polystyrene resin A and the same • procedure as in example 1, a rekkV "s-compound plate with a cross section of 110 mm x 350 mm, axial cell dimensions-

ratio y/x and y/z of 1.1 to 1.25 resp. 1.1 to 1.17, while the cell size in the Y direction and the mass density d are varied in the range from 0.07 to 1.6 mm resp. 21.5 to 77 kg/m". The foam

types lighter than about 21 kg/m<3> were subjected to a secondary expansion by treatment with steam of 100°C for two to six minutes, so that the bulk density became from 15.5, to 20 kg/m"'. The skin coating was removed from each foam board which received a cross-

section of approximately 100' mm x 300 mm and a length of 2000 mm.

These resulting foam sheets were mechanically compressed by flexibilization in the X direction first and then in the Z direction by

use it in fig. 5 shown equipment and the typical previously described

ne conditions, including extensive aging one day after extrusion.

Pre-drawn examples 201-212; Reference examples R 201-206

. As a result of the compression process, flexibility had

serted foam sheets according to Preferred Examples 201-212 and Reference Examples R201-206 almost constant cell shape with axial cell dimension ratios y/x and y/z from about 1.2 to 1.4. These flexible sheets were then assessed by standard

procedures, and typical results are shown in Table 2.

Based on the typical results shown in Table 2, the mass densities D were plotted in the diagram of Fig. 6B against the average cell dimensions y in the Y direction, where the coordinates for the foam test pieces that were assessed as excellent and good in table 2 are marked with 0 and. o, while those that were assessed as unacceptable are marked with an X.

As shown in the diagram in fig. 6B, the foam types according to the present invention must have such an average cell dimension y in the Y direction in mm and such mass densities D in kg/m<3> that the drop in the pentagonal area defined by the five coordinates (1.0,

■35), (0 .25 , 100), (0 .05 , 100), (0.05 , 26 .5) and (1.0, 20) and preferably in the pentagonal area defined by the coordinates (0, 8, 50), (0 .25, 93), (0 .07, 28 , 5) and (0.8, 2.3.5).

In other words, the foam types that the present invention is considering using must have such a mass density D (k.g/m'<J>) and average cell dimension y (mm) in the Y direction that they satisfy the following formulas: and preferably;

Example 3 Flexibilisation time

It is common practice for rigid thermoplastic foam sheets to be aged for at least several weeks before use to stabilize the okum structure. During the development of the flexibilized foam sheets for use as cryogenic insulation, it was discovered that the aging of the flexibilized foam by the flexibilizing compression greatly affected the properties of the. it t.esul-I. is end foam.

Using foam sheets extruded from polystyrene resin A and cutting standard 25 mm and .100 mm thick pieces, the effect of flexibilization time was investigated after flexibilization in both one and two directions. Tyoic results are shown schematically in fig. 7 and 8, where the A series show flexibilization in one direction (the X direction) and the B series show flexibilization in two directions (the X direction and then the Z direction),

A. Flexibilization in one direction

Fig. 7A shows the relationship between the extension Ex in X-ret-

the breaking time for the flexibilized foam boards and the aging period for the original foam board after the expansion,

while fig. 8A shows the relationship between water vapor permeability in the teat and the aging period before the flexible serina. It is obvious that in order to obtain the improved properties regarding elongation and water vapor permeability according to the present invention, it is necessary

recommended that the aging period for the foam sheets prior to the compression flexibilization should not be more than 10 days (240 hours) and preferably not more than 3 days (72 hours).

B. Flexibilization in two directions

Fig. 7B shows the relationship between the extension orocent Ex

in the X direction at failure for foam boards that are flexible. in

-two directions and the aging time of the extruded foam sheets.

Note that the effects of aging on the elongation rate

in the X direction and the Z direction become essentially the same. They up-

flowable extruded foam sheets had a density now of about 2 7

kg/m3, a thickness of approximately 100 mm, and the average cc1ledimen-

tions in the X, Y, and Z directions were approximately 0.55 mm, 0.72 mm, resp. 0.58 mm. After cutting to a thickness of 25 mm, the foam boards were first subjected to a single 37 percent compression in the X direction and then in the Z direction with varied aging times. The elongation percentage Ez in the Z direction at break lies* r oå om-

trained 80 to 90 percent of the elongation percent Ex in X-retnin-

gene in case of breakage. In fig. 7B is the elongation percentage v r- .1 fracture represented as the elongation percentage Ex in the X direction at fracture.

Fig. 8B shows the relationship between the water vapor permeability Py

Por flexible foamolates and the aging period, for the foam material after its expansion. The foam boards have the same density and the same cell dimensions as indicated above. Try-

pieces of approximately 25 mm thickness were cut out and subjected to a 20-37 percent compression performed three times in each direction.

The resulting foam sheets have an elongation percentage o; in the X direction at a break of approximately 20 percent and an extension

percent Ez in the Z direction of about 16 percent.

It is again clear that in order to achieve the desired properties

per, the foam sheets must be flek-ibilized while they are freshly extruded, i.e.

within 10 days and preferably within 3 days of extrusion

gene or the expansion. This applies in particular to relatively dense foam boards, of which the 25 mm thick test pieces used in the above experiments are representative. The optimal time lies within the range from approximately 0.25 to 240 hours and will of course depend on the special properties of the original foam sheets and the desired results.

Example 4 Water vapor permeability

Critical to low-temperature insulation is the foam's ability to form an effective barrier against water vapor passage from the outer to the inner surface of the insulation.

Å. Foam flexible in one direction: Preferred Examples 121-132 + Reference Examples R121-126

Using the same equipment and methods, flexible foam sheets were expanded from PS resin A under controlled conditions, so that the resulting foam sheets had densities D ranging from about 22.5 to 51 kg/m , a cell dimension y in Y direction in the range from about 0.07 to 1.0 mm and cell dimension ratios y/x and y/z from about 1.35 to 2 resp. 1.1 to 1.3. Then, the resulting foam sheets were cut into a 100 mm square cross-section and 4000 mm length, and after aging for one day, they were compressed in the X direction. Typical values and properties, also including the water vapor permeability for

these flexible foam sheets are given in table 3.

Based on such typical results as shown in table

3, the flexible foam sheets according to the present cpp invention must have a water vapor permeability of 1.5 g/m 2.hour or less, as it is examined according to the water method according to ASTM C-355.

Figs. 1A and 1B are photomicrographs (magnification: 50x) of preferred example 123 polystyrene foam showing closed cells distributed as seen in the X, Y and Z directions of Figs. 3. Note that the flexible foam according to the present invention' has a uniformly structured anisotropy, where the ripples in the cell walls in the YZ plane (fig. IA) are significantly fewer than in the XZ and XY planes fig. IB and 1C). As the foam cells are very small. and has a polygonal shape, it is very difficult to define the distribution and location of such ripples precisely-. When considering the relationships between Ex, E and the water vapor permeability Py in the Y direction and with reference to fig. 1, these conditions will constitute fairly accurate, structural parameters for the ripples, including their type, their location and distribution (fig. 9A). As it also appears from fig. 9A, where the water vapor permeability is plotted against the cell shape of the foam, it is desirable to have the cells oriented along the Y axis, preferably with an average y/x 'cell dimension j,on_s ratio pl 1.2 to 3.

B. Foam flexible in two directions: Preferred Examples 221-227 + Reference Examples R221-225.

By using the same PS resin A, the same equipment and the same method of remaang as in example 1, foam sheets with the same cross-section were extruded and expanded to a density oå. 27 kg/l m"-x' or 50 kg/m<3>, and with mean cell dimensions in the Y direction r. to 0.51 m'i or 0.11 mm and with y/x of 1.20 or 1.15 or "y/z 1.25 or 1.20. These foam boards were first compressed in the X direction and then in the Z direction during flexibilization using the equipment shown in fig. 5. Then the foam densities D and other values were measured, including the permeability Py in the Y direction in the r'; in this way, biaxially flexible foam materials, the Endrin genes in the thermal conductivity in the Y direction and the cryogenic resistance

-160°C and -196°C in the X direction and Z direction were also observed Tynic results are shown in Table 4.

Table 4 shows that the foam boards according to the invention must have

a water vapor permeability Py in the Y direction equal to or less than 1.5 g/m 2.hour to prevent or reduce the loss of heat-insulating properties during long-term use. The water vapor permeability should preferably be 1.0 g/m or less to ensure a higher thermal insulation ability.

For use under such harsh conditions as are found in liquid nitrogen gas tanks and to ensure improved insulation properties over long periods of time, the preferred foam types according to the present invention must also satisfy the following conditions:

Figs. 2A, 2B and 2C are photomicrographs (magnification: 50x) of the flexible polystyrene foam according to the preferred example 22 3 shows the closed cells, seen in the X-, Y- and Z~ directions, as shown in fig. 3. Note that the foam is characterized by structurally anisotropic cell walls. These - which are visible in the YZ and Xy planes in fig. 2A and 2G are generally wavy in one direction only, namely in the Z direction and the X direction, but not in the Y axis direction.

Such anisotropically distributed cell wall undulations in combination with the foam density as well as the dimensions and shape of the cells are important structural parameters for the foam boards according to the present invention, seen against the background of the previously mentioned ratio between Ex and Ez, the ratios between the axial elongation percentages at break (Ex/ Ey, Ez/Ey) and the water vapour<p>ermability in the Y direction which represents the distribution and directions of such ripples (Fig. 9B). As also shown in fig. 9B, where the water vapor permeability is plotted against the cell shape of the foam, it is desirable that the cells in the foam are arranged along the Y axis, preferably with an average y/x cell dimension ratio of 1 or higher.

Example 5 Cryogenic insulation

A. Foam flexed in one direction

An experiment has surprisingly indicated that foam sheets wrapped around a steel drum which is heated to approximately 80°C have the desired and improved elongation properties and water vapor barrier properties, and these foam sheets can be shaped to the curvature of the drum and can be attached to this drum. Nevertheless, the winding does not require any greater force and causes only a minimal reduction in the thermal insulation properties.

Table 5 shows • results of experiments on yet another group of preferred examples of foam types according to the present invention and of several reference examples. As these assessment values are mainly representative of the flexibility, applicability on curved surfaces, workability with glue, cryogenic insulation properties and other characteristics, which are required of such foam materials, table 5 gives an overall assessment for the practical applicability of such foam materials.

In order to reduce the multiaxial stresses in the foam materials after their application and to provide an effective improvement of the thermal insulation properties, two or more foam layers can be bonded together, so that the resulting foam sheets exhibit biaxial extensibility, or they can be coated with rnetsll foils or combined with synthetic plastic films, which have good properties as gas barriers.

B. Foam flexible in two directions.

In order to determine the applicability of the foam material for curved surfaces, such as pipes and cylindrical or spherical tanks, the workability, including the flexibility and formability, as well as the cryogenic thermal insulation ability, selected foam materials, namely foam types according to the preferred examples 2 22-225 and according to the reference examples R221, R223-225, laid on an outer steel tube with an outer diameter of approximately 114 mm as a typical example of cylindrical tubes with very large curvature. Foam boards were cut to thicknesses of 25, 37.5 or 75 mm and laid on in one, two or three layers. to achieve a total thickness of 75' mm. The longitudinal and circumferential seams of the semi-cylindrical foam board parts, which are applied in layers, butt against. each other, while foam board parts with a thickness of 75 mm were double folded.

The flexibility, the heat formability of the bent shapes, the cryogenic heat insulating properties and the crack or crack mct condition of these plates were tested, and typical examples are given in table 6..

The synthetic foam resins of the present invention which have greater extensibility in two axial directions exhibit excellent bendability, hot formability and applicability to pipes having a small diameter. They can be easily applied to small diameter pipes and they are easy to hot form into the bent shape. Due to the fact that these foam materials do not so easily form cracks during the bending operation or under cryogenic conditions, furthermore the foam materials according to the present invention can be excellent cryogenic thermal insulation materials which avoid water vapor condensation even at -196°C, and which can generally be used for pipes, cylindrical and spherical tanks.

Although the reference foam sheets which are only comoromized in the X and Z directions have satisfactory bendability and hot formability, they are not fully satisfactory as cryogenic thermal insulation because they may be subject to fracture under cryogenic conditions due to crack propagation in the circumferential direction of the pipe or in other directions. Such cracks form because these types of foam do not have such great stretchability that they can absorb stresses caused by sudden changes between room temperature and cryogenic temperature.

Example 6 Thermoplastic foam resins The improved flexibilization process can be used for a number of thermoplastic foam resins, which are both extruded and expanded.

A. Commercial PS resin A is a heat-polymerized polystyrene resin having an intrinsic viscosity of approximately 0.83 dissolved in toluene at 30°C and containing 0.20 weight percent volatile residues, comprising styrene monomer, and 0.87 weight percent oligomers, comprising styrene trimers. Blends with other polystyrene resins having more styrene monomer and trimer were flexified, and typical results are shown in Table 7. For such hot polymerized polystyrene resins, the preferred resins in the flexibilized foam boards are those containing 0.3 weight percent or less of volatile residues, comprising styrene monomer and 0.5-1.5% by weight of styrene oligomers, comprising trimers.

B. Instead of the polystyrene foam types used in the preceding examples, two commercially available polyvinyl chloride foams (Klegecell<®> 33 produced by Kanegafuchi Chemical Co., Ltd. and Rockecell Board®, produced by Fuji Kasei Co., Ltd.) and a methyl methacrylate resin foam (experimentally produced by Asahi-Dow Limited, which was cut into 50 x 600 x 900 (mm), 25 x 600 x 900 (mm) and 50 x 300 x 900 (mm).

The resulting flexibilized foam boards were tested and assessed, where the typical results are shown in Table 8. The present invention is thus also applicable to foams that are expanded from polyvinyl chloride resins and comprehensive mixtures of these with inorganic materials, methyl methacrylate and other similar resins apart from polystyrene, and the resulting flexible foam sheets satisfy the requirements according to the present invention. C. A batch of preformed polystyrene beads having a bulk density of 11.6 kg/m<3> was placed in a mold and steam-heated for approximately 40 seconds under a pressure of 3 kg/cm 2 .

The resulting foam was aged at 70°C for 12 hours. It had a density of 10.9 kg/m<3> with x of 0.33,- y of 0.31' and z. of 0.32. Three 350 mm cubes were cut from the central parts using an electrically heated wire cutter.

A specimen was flexed in the X direction to 90 percent of its original volume by applying a 40 k<q>/cm<2> pressure in a 50 ton press. The compression was repeated six times and the pressure ceased immediately after its application. The compressed foam was given a size of 350 x 350 x 262 mm with a density of 14.5 kg/m<3>.

The other test pieces were flexed in a similar way in two and three directions. All were subjected to standard tests and they did not satisfy one or more of the desired results which the present invention aims to achieve. It should also be noted that none had the required foam density.'

Claims (9)

1. A unidirectional flexibilized foam plastic sheet of substantially closed-cell polystyrene resin, generally having a rectangular shape defined by the three-dimensional coordinates X, Y, and Z and having an anisotropic rippled cell wall structure formed by a partial crushing of the foam in the direction perpendicular to the direction of flexibilization , characterized in that the foam has (1) a mass density of 20 to 60 kg/m<3>, (2) an anisotropic cell structure oriented in the Y direction and an average cell dimension y of 0.05 to 1.00 mm, (3) mean cell dimensions x, y and z which satisfy the following conditions: y/x and y/z > 1.05, (4) an elongation at break (Ex) in the X direction of 7 to 70 percent, (5) a water vapor permeability (Py) in the Y direction of not more than 1.0 g/m<2>#hour by the water method according to ASTM C-355 and (6) a thickness of 10 - 300 mm. 2. A bidirectionally flexible foam plastic sheet of mainly closed-cell thermoplastic resin, which generally has a rectangular shape defined by the three-dimensional coordinates X, Y, Z and has an anisotropically rippled cell wall structure that is most rippled in the XZ plane, characterized in that the foam has: (•1.)) em1, mass density of 20 to 100 kg/m3 , ((2!)) other cell dimensions x, y, z measured in the X-, Y- and Z-axis direction which satisfy the following conditions: y = 0.05 - 1.0 mm and y/x and y/z > 1.05, (3) axial elongation at break (Ex, Ey, Ez) satisfying the following conditions: Ex > 1.8 Ey and Ez < 8.3 Ey, (4) a water vapor permeability in the Y direction of not more than 1.5 g/m2.hour according to the water method according to ASTM C-355 and (5) a thickness of 10 - 300 mm. 3. Flexible foam of thermoplastic resin according to claim 2, characterized in that the resin is polystyrene. 4. Flexible foam a<y> polystyrene resin according to claim 1 or 3, characterized in that the polystyrene resin contains 0.3 weight percent or less of volatile residues, comprising styrene monomer and 0.5 to 1.5 weight percent styrene oligomers. 5. Method for flexibilizing a rigid, substantially closed-cell foam plastic sheet generally having a rectangular shape defined by three-dimensional coordinates X (length), Y (thickness), Z (width) and the YZ, XZ and XY planes normal to these by a partial crushing of the foam board in a direction normal to the direction of the desired flexibility, characterized by the combination of the following features: A. that a newly expanded foam board is selected which has (1) a mass density of from 20 to 100 kg/m<3 >, (2) an anisotropic cell structure oriented in the Y axis direction with an average cell size y of from 0.05 to 1.0 mm, (3) a compressive strength in the Y direction of at least 1.8 kg/cm<2> and (4) a thickness of 10 - 3 00 mm, B. that this foam sheet is compressed within 0.1 to 240 hours after expansion in a short delimited compression zone with a maximum length of 300 mm for a period of at least one second to form a directionally flexible foam board, and then C. that a directional flexible is recovered foam board having (1) anisotropic rippled cell wall structure, where the ripples lie in the flexibilization direction, (2) average cell dimensions x, y and z measured in the X, Y and z directions that satisfy the following conditions: y = 0.05 - 1, 0 mm and y/x and y/z > 1.05, (3) an increased elongation at break in the flexibilization direction, and (4) a water vapor permeability in the Y direction not greater than 1.5 g/m< 2>.time.determined by the water method according to ASTM C-355. 6. Method according to claim 5, characterized in that the foam board is flexible within 72 hours after the expansion. 7. Method according to claim 5, characterized in that the thermoplastic resin is polystyrene. 8. Method according to claim 7, characterized in that the polystyrene resin contains 0.3 weight percent or less of volatile residues, comprising styrene monomer and 0.5 to 1.5 weight percent of styrene oligomers. 9. Method according to claim 7, characterized in that the polystyrene foam resin is compressed first in the longitudinal (X-axis) direction and then in the transverse (Z-axis) direction to give a polystyrene foam sheet which is flexible in two directions.