Shaping anodic aluminum oxide sheet

Porous anodic aluminum oxide sheet, when removed from its aluminum metal substrate, is a useful filter but rather brittle. It can be formed into a desired shape by flexing in a direction transverse to the direction in which the metal substrate was rolled. Or the sheet can be wetted to make it more flexible and then formed by flexing in any direction. The formed sheet can be heat-set in a desired shape by heating at 200.degree. C. to 650.degree. C.

When an aluminium metal substrate is anodized in an electrolyte such as 
sulphuric acid or phosphoric acid, an anodic oxide film is formed on the 
surface. This film has a relatively thick porous layer comprising 
regularly spaced pores extending from the outer surface in towards the 
metal; and a relatively thin non-porous barrier layer adjacent the 
metal/oxide interface. As anodizing continues, metal is converted to oxide 
at the metal/oxide interface, and the pores extend further into the film, 
so that the thickness of the barrier layer remains constant. The 
cross-section and spacing of the pores and the thickness of the barrier 
layer are all proportional to the anodizing voltage. 
It is possible to spare the anodic oxide film from the metal substrate. If 
the barrier layer is also removed, there remains a porous anodic aluminium 
oxide sheet. Such sheets are useful as filters for example for 
desalination of salt water, dewatering of whey, or for dialysis. Other 
uses include bacterial filters for cold sterilization, and gas cleaning. 
EPA No. 178831 describes a method of making a porous anodic aluminium oxide 
sheet by the steps of anodizing an aluminium metal substrate; slowly 
reducing the applied voltage under controlled conditions to a level 
generally below 3V; carefully lifting the anodic oxide sheet from the 
metal substrate; and drying the sheet. Depending on thickness, the 
resulting sheet has quite good tensile strength but rather poor bending 
strength, and is fairly fragile and brittle. 
For high performance, filter devices need to have a high filter area per 
unit volume. This is not easy to achieve with flat sheets of filter 
material. So it is usual to fold, pleat or roll filter sheets into 
spirals, in order to increase the surface area per unit volume. But 
conventional techniques cannot be applied to anodic aluminium oxide sheets 
on account of their fragility. 
This invention arises in part from the discovery that if such a sheet is 
thoroughly wetted it becomes much more flexible than when dry. This 
discovery was unexpected. The invention thus provides in one aspect a 
method of forming into a desired shape an anodic aluminium oxide sheet, 
which method comprises wetting the sheet, forming the wetted sheet into 
the desired shape, and drying the sheet in that shape. 
The invention further arises in part from the discovery that the sheet is 
better able to tolerate (without cracking) flexing in one direction than 
in another. This discovery applies to both wet and dry sheets, although 
wet sheets are more able to tolerate flexing in any given direction than 
the dry ones. As noted above, the aluminium oxide sheet is made by 
anodizing an aluminium metal substrate, which substrate will itself have 
been formed by elongation, e.g. by rolling, in a particular direction. It 
turns out that the anodic aluminium oxide sheet, after seperation form 
this substrate, is more tolerant to (without cracking) flexing in a 
direction transverse, e.g. at right angles, to the elongation direction. 
The invention provides in another aspect a method of forming into a 
desired shape an aluminium oxide sheet, said sheet having been made by 
anodizing an aluminium metal substrate which had previously been elongated 
in a particular direction which method comprises flexing the sheet in a 
direction, transverse to the particular direction so as to form the sheet 
without cracking into the desired shape. This aspect of the invention 
applies to both wet and dry sheets. The transverse direction is preferably 
at least 30.degree. C. to the elongation direction. 
Although other methods are described in the literature, the starting sheet 
is preferably made by the method of EPA No. 178831. The sheet is generally 
wetted with water since other liquids may be a fire hazard, and at ambient 
temperature. 
The sheet is preferably formed into the desired shape by rolling. In this 
context, rolling means curling the membrane e.g. into a tubular or spiral 
shape. To avoid bending or fracturing the sheet during this operation, a 
cylindrical former is preferably used. To ensure adhesion of the sheet to 
the former, an adhesive may be applied to the surface of the former. 
Alternatively, if the former is hollow and perforated, the anodic oxide 
sheet may be held in place by reducing the pressure within the former. 
After the rolling operation has been completed, the sheet may be 
temporarily held in position on the former by means of clips or clamps. 
Alternatively the sheet may be deformed to a desired shape by the 
application of a pressure differential across the sheet, e.g. by applying 
a vacuum to one side of the sheet or by press forming. 
The sheet is preferably from 10 to 80 microns, particularly from 15 to 70 
microns thick. Thick sheets tend to be rather inflexible even when wetted. 
A single thickness of sheet may be rolled round a former. More usually, 
several thicknesses of sheet are wound in the form of a sprial, with a 
spacer between adjacent layers of anodic oxide sheet. For this purpose, a 
wetted sheet may be laid on a flat surface, a spacer sheet laid on top of 
it, and the two rolled together round the former. Alternatively, a stack 
of several sheets, alternately anodic oxide sheets and spacer sheets, may 
be laid on the flat surface and rolled up together. When the anodic oxide 
sheet is so thin as to be fragile on its own, it may be used on a porous 
support, in which case a stack of alternating (support plus anodic oxide) 
sheets and spacer sheets may be built up. 
The deformations described are generally elastic, that is to say, when the 
deforming stress is removed the sheet returns to substantially its 
original shape. This is true even if the sheet is deformed when wet and 
then dried at low temperature in the deformed state. Various techniques 
may be used to hold the formed sheet in its desired shape, including clips 
supports, adhesives and heat staking. When the desired shape includes 
overlapping portions of sheet, the formed sheet can be held in shape by 
means of an adhesive applied between the overlapping portions. The sheet 
may advantageously be formed in the presence of a solution of phosphoric 
acid, which may confer benefit in particular by acting as an adhesive on 
drying. 
This invention is further based on the discovery that the formed sheet can 
be heated under conditions to heat-set it in the desired shape. If the 
formed sheet, in the wet or dry state, is heated, perferably at from 
200.degree. C. to 650.degree. C. for from 5 to 500 minutes, it is found to 
retain its shape without the need for clips, supports or adhesive. Longer 
times and/or higher temperatures could be used. 
The shaped sheet can be used in position on a former which provides 
mechanical strength in use. Alternatively, the former can be removed, but 
some other support is then generally required to avoid risk of fracture. 
A filtration device already exists which incorporates rolled membrane, but 
the membranes used are organic and polymeric and are may not be naturally 
brittle. The existing device is known as a spiral wound membrane module; 
membranes are placed as a sandwich in a porous support and then wound in a 
spiral configuration. A relatively high surface area can be installed per 
unit volume and costs are relatively low. The use of anodic aluminium 
oxide sheets to replace organic polymeric membranes in this application 
may be advantageous because anodic oxide sheets have high porosity and a 
narrow pore size distributor giving high flow rates and sharp cut-off.

EXAMPLE 1 
In order to perform this operation, an anodic aluminium oxide sheet (60 
mm.times.100 mm and 30 microns thick) was placed in a shallow tray of 
deionised water until it was completely wet. It was then removed carefully 
and placed on a clean flat surface, viz. a glass plate, taking care not to 
fold or break the sheet in the process. One edge of the membrane sheet was 
carefully lifted and subjected to a very gentle rolling movement. 
Extremely slight pressure was applied to the sheet roll as it formed. When 
the complete sheet was formed into the tubular arrangement, the loose end 
was secured in its final position. The sheet in its tubular form was then 
placed carefully in an oven at 40.degree. C. and left until completely 
dry. The clips securing the roll were then removed and the tube retained 
its shape. It is possible then to fabricate a holder for this roll for use 
in filtration processes. 
EXAMPLE 2 
The experiment described in Example 1 was repeated with two sheets of 
anodic oxide 30 microns thick produced by anodising in phosphoric acid. 
Both sheets were rinsed thoroughly with de-ionised water. The first sheet 
was wetted with de-ionised water before being formed into a tube. The 
second sheet was wetted with dilute phosphoric acid before being formed 
into a tube. Both tubes were held in shape by a constraint and dried at 
40.degree. C. for 17 hours. 
On removing the constraints the well rinsed membrane was observed to 
recover its original flat shape. The second sheet, which had been wetted 
with dilute phosphoric acid, retained its tubular shape. 
It is believed that the phosphoric acid makes the membrane surface slightly 
sticky and on drying the overlapping layers become glued together 
retaining the tubular shape. 
EXAMPLE 3 
Well rinsed 30 micron thick asymmetric membranes approx. 10 cm.times.4 cm, 
produced by anodising in mixed acid, were soaked in de-ionised water for 
15 minutes at 20.degree. C. and then rolled to make a cylindrical shape 
with the edges of the membrane overlapping. The rolled membranes were 
retained in a beaker and heated in an oven to temperatures between 
100.degree. and 650.degree. C. for times between 60 and 480 minutes. On 
cooling to room temperature the membranes were removed from the beaker and 
the resulting shape observed. At temperature below 200.degree. C. there 
was no retention of the cylindrical shape; the membranes simply returned 
to their original flat sheet. Heating for several hours at 200.degree. C. 
resulted in some retention of shape and good shape retention was obtained 
by heating for 1 hour at 400.degree. or 650.degree. C. (see Table 1). 
TABLE 1 
______________________________________ 
Former Drying 
Radius Temp Time Shape 
(mm) (.degree.C.) (min) Retention 
______________________________________ 
15 100 60 None 
15 100 240 None 
15 100 480 None 
15 200 60 None 
15 200 240 Poor 
15 400 60 Good 
15 650 60 Good 
15 650 60 Good* 
______________________________________ 
The cylinders heated to 400.degree. or 650.degree. C. retained their shape 
after exposure to cold or to hot (60.degree. C.) deionised water for 
several hours indicating that a permanent change in shape was brought 
about by these heat treatments. 
EXAMPLE 4 
Dry, well rinsed 30 and 60 micron thick symmetric and asymmetric membranes 
approx. 26 cm.times.12 cm produced by anodising in phosphoric acid were 
rolled round a 24 mm diameter form. The membranes were formed successfully 
when the long axis of the cylinder was at 90.degree. or 45.degree. or 
30.degree. the rolling direction of the sheet aluminium on which the 
anodic oxide was grown. Cracking occurred when the cylinders were formed 
with their long axis parallel to the rolling direction of the aluminium 
sheet. 
A cylinder formed by rolling the dry film was secured and heated to 
650.degree. C. for 1 hour. On cooling and releasing the constraints 
holding the cylinder the shape was retained. 
EXAMPLE 5 
This example illustrates vacuum forming of the anodic membrane to make a 
corrugated sheet. 
The die used for forming was made from a flat aluminium plate into which 
two sets of grooves were cut. In one set of grooves the ridges were 10 mm 
apart and the valleys 4 mm deep. The other set was finer having ridges 5 
mm apart and only 2 mm deep. The grooves were generally of sine wave form 
and the tops of the ridges of both sets were in the same plane. 
Forming was carried out by laying an anodic membrane 10 cm.times.7 cm 
across both sets of grooves. A flexible plastic vacuum bag was placed 
around the assembly and the air evacuated slowly over a period of about 1 
hour by means of a rotary vacuum punp. As the air was removed the vacuum 
bag collapsed down onto the membrane pushing it down into the die. 
Membranes were tested either dry or after soaking in deionised water for 15 
minutes at 20.degree. C. 30 micron thick asymmetric membranes produced by 
anodising in phosphoric acid or 60 micron asymmetric membranes produced by 
anodising in mixed acids were used. 
In all of the tests, the wet membrane took up the shape conforming to the 
profile of the coarse and the fine grooves without cracking. The dry 
membranes cracked along the ridges or bottoms of the valleys. 
When the vacuum was released the unbroken membrane recovered its original 
shape. 
EXAMPLE 6 
This example illustrates press forming of the anodic membrane. 
An anodic membrane 10 cm.times.7 cm was laid flat on a bed of fine zirconia 
powder. The die used in Example 5 was placed on the membrane and pressed 
down with a 4 kg weight while the assembly was heated to 400.degree. C. 
for 1 hour. 
Wet or dry membranes similar to those used in Example 5 were pressed 
formed. In all cases the membranes retained a slight imprint of the die on 
cooling but due to the small deformation imposed under these conditions it 
was not possible to make an exact replica of the die.