Pipes and tubular conduits – Flexible – Corrugated
Reexamination Certificate
2001-09-20
2002-05-28
Brinson, Patrick (Department: 3752)
Pipes and tubular conduits
Flexible
Corrugated
C138S122000, C138S177000, C264S286000
Reexamination Certificate
active
06394144
ABSTRACT:
The present invention relates to a polytetrafluoroethylene (PTFE) tube and more particularly to a PTFE tube for a flexible hose. In particular the invention relates to a PTFE tube having a smooth bore for use in the production of a lined hose assembly further comprising hose braids, external hose protection and end fittings.
It should be noted that there are two basic types of internal tube configuration;
smooth bore tubes, as their name suggests, have a substantially convolution free internal surface;
in contrast, internally convoluted tubes, as their name suggests, comprise a number of distinct peaks and roots.
Of course smooth bore tubes are often not totally devoid of bumps and indentations and may show rippling. This is however in sharp contrast to the induced peaks and roots of an internally convoluted tube.
PTFE is a unique material and is favoured for applications in the transport of foodstuffs and chemicals because of its chemical resistance and non-stick nature. However PTFE is not naturally elastic.
Producing a flexible PTFE tube for certain applications, particularly high pressure applications, where fluids, more particularly gases and vapours, are pumped through the tube has proved difficult. Indeed, it had previously been thought that many convoluted PTFE tubes would not be suitable for such applications because the “thinning” of the walls to produce “flex” was expected to result in increased permeation to fluids.
To reduce permeation one or more of the following techniques have hitherto been employed:
1. Wall thicknesses have been increased;
2. Higher grade polymers have been used; or
3. Polymers have been processed to have increased crystalinity.
Increasing the wall thickness decreases the flexibility of the finished product as well as increasing its weight and cost.
Increasing crystalinity increases the flexural modulus of the material thus decreasing the flexibility and this also incurs a reduction in flex life.
Du Pont, for example, define crystalinity as being low (50%), moderately high (72%) or very high (82%). At low crystalinity the product has a flexural modulus of 54,000 psi and a relative permeability to CO
2
gas of 6; at moderately high crystalinity the product has a flexural modulus of 150,000 psi and relative permeability to CO
2
gas of 0.8 and at a very high crystalinity the product has a flexural modulus of 170,000 psi and a relative permeability to CO
2
gas of 0.2.
Most corrugated products are made by a process which convolutes or concertinas the product and have walls which are substantially uniform in thickness throughout. Typical processes include those described in GB 1543586 and GB 2293222.
EP 474449 B1 on the other hand discloses a corrugated plastics tube which has been subject to a compression force to displace material in the root region. It is characterised in that the compression force applied was sufficient to take the plastics of the tube, which was at a temperature below its melt temperature, beyond its elastic point. This can be achieved at any temperature below the melt temperature and the patent makes no specific teaching in this regard. Furthermore, the patent relates to plastics in general and is directed to producing flexibility. It is not particular to PTFE (although PTFE is specified) and it does not address the problem of producing tubes with improved permeability resistance to gases.
In contrast the present invention, which is particular to PTFE, teaches that a novel product is obtained by a process comprising
1. subjecting the PTFE tube to a deformation force at a temperature at or above the gel transition temperature of PTFE to produce constrained convolutions having a thinned wall W
1
; and
2. cooling the PTFE tube to below the gel transition temperature whilst continuing to constrain the deformations having the thinned wall W
1
until the convolutions having the thinned wall W
1
have become stable.
This product is characterised in that the convoluted PFTE tube has an improved resistance, of greater than 7.6%, to permeation by comparison with the non-convoluted tube, the comparison being made between tubes of (i) equal nominal bore ID; and (ii) equal weight of PTFE per unit length.
This improved resistance to permeation is indicative of the fact the product processed in this manner has a different form to one not so processed. This can be confirmed by way of the test procedure set out in the specific description.
Surprisingly, the applicants have discovered that by processing the PTFE, which term includes modified PTFE, in a particular manner they are able to reduce permeation rates for a given thickness of PTFE. That the PTFE processed in this manner has a changed form can be characterised by amongst other things, its improved resistance to permeation and increased tensile strength.
According to a first aspect of the present invention there is provided a PTFE tube comprising external roots and peaks which tube is obtainable from a non-convoluted tube having an original wall thickness W
0
and an internal diameter ID by a process in which a region of the tube is thinned to provide external convolutions with a root wall thickness W
1
, characterised in that the convoluted PTFE tube has an improved resistance to permeation of greater than 7.6% by comparison with the non-convoluted tube, the comparison being made between tubes of (i) equal nominal bore ID; and (ii) equal weight of PTFE per unit length.
Preferably the PTFE tube has a smooth internal bore.
In one embodiment the smoothbore has a rippled appearance.
According to a further aspect of the present invention there is provided a method of producing a PTFE tube comprising external roots and peaks from a non-convoluted tube having an original wall thickness W
0
comprising:
1. subjecting the PTFE tube to a deformation force at a temperature at or above the gel transition temperature of PTFE to produce constrained convolutions having a thinned wall W
1
; and
2. cooling the PTFE tube to below the gel transition temperature whilst continuing to constrain the deformations having the thinned wall W
1
until the convolutions having the thinned wall W
1
have become stable.
Preferably W
1
is less than 25% of W
0
.
More preferably W
1
is about 20% of W
0
. In a preferred embodiment the PRFE tube is produced on a mandrel of substantially the same size as the internal diameter of a plane cylindrical PTFE paste extruded tube such that the resulting tube is a smoothbore, externally convoluted, tube. The resulting smoothbore tube has a rippled appearance.
That the deformation has become stable can be characterised by an increase in tensile strength indicating that the deformation is a “yield” deformation. The deformation can be further characterised in that it is reversible. i.e. when the deformed material is reheated to at or above the gel transition temperature without a restraining force in place, it returns substantially to its original form.
It is also possible to determine whether or not the PTFE was deformed at a temperature above or below the gel transition temperature. A tube deformed below the gel transition temperature will revert partially or substantially to its original form at temperatures below the gel transition temperature whereas one deformed at or above the gel transition temperature will only revert substantially to its original form at or above the gel transition temperature.
The increase in tensile strength can be seen by conducting a simple test. A longitudinal section is taken from a convoluted tube prepared in accordance with the invention and is gripped on either side of a root. It is then pulled apart until the section breaks at the root. By first determining the thickness and width of the root and noting the force applied to break the tube at its root the breaking force per cross sectional area can be calculated. Another section of the tube is then heated to above the gel transition temperature so it reverts to its starting conformation and the section is then subjected to the same test i.e. it is pulled along the longitudinal axis of the tube.
Brinson Patrick
Townsend and Townsend / and Crew LLP
Whitworth John Andrew
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