Note: Descriptions are shown in the official language in which they were submitted.
~S2~
The present invention relates to the incorporatiGn of additives
into cross-linked polyolefins and has particular re~erence to the
~ inclusion of stabilizers, surface fricti.on reductants and the like.Many dlfferent types of heat stabilizers and additives in
general for polyolefinic materials, e.g., polyethylene, can be emp].oyedO
Some of these are employed to stabilize the material during processing
while others are added to the polymer mostly to ensure long life of the
finished products and also to enhance surEace and other properties of
the materlal. Where manufactured cross-linked polyolefins, e.g. cross-
linked polyethylene, are involved, the mixing of many additives with a
cross-linking initiator or cross-linking agent, e.g. a peroxide cross-
linking agent, has the effect of (a~ diminishing the effectiveness of
the additive, and (b) makes it necessary to use more peroxide to obtain
the same degree of cross-linking.
Furthermore, some reaction can occur between the pero~ide
cross-linking agent and the additive which can, in extrem~ circumstances,
cause the generation of gases during the cross-linking reaction thus
resulting, during formation and during the cross-linking process, in the
formation of small blisters in the final product.
In consequence, it has been necessary, hitherto, to use at the
most 0.15% to 0.2% by weight of additive for inclusion in the cross-
linkable material prior to the cross-linking and forming since amounts
in excess of this are liable to result in one or more of the disadvan-
tages referred to above.
Where the additive is a stabiliæer for imparting heat stability
to the cross-linked material, it is necessary to include, in general, a
greater proportion than 0.2% by weight oE stabilizer within the cross-
linked material in order to impart satisfactory heat stabilization.
Furthermore, with the advent of polyethylene pipes as described ~ ~
in British Patent Specification No. 1,158,011 and the use of such pipes ~ .
for hot water systems and the like, the inclusion of a sufficient quan-
tity of stabilizer for the solution of this particular problem of high
S~ L3
temperature stability has become Inore urgent. ~hi:Le pipes of
unstabilized cross-linked'polyethylene have extremely good properties,
there is a certain limit above which it is necessary to stabilize the
material of the pipe and this, to some extent, limits the use and areas
of use of the material. It has been'found, however, that if adequate
stabilizer can be introduced into the cross-linked material without
other side effects, then it is possible to use cross-linked polyolefinic
materials for fluid temperatures between the range 100C. to 120C.
Furthermore the ability to include in an article additives
: 10 which reduce surface Eriction property thereof makes such material parti-
cularly useful for such diverse uses as ski soles and'boat hulls.
According to an aspect of the present invention, therefore, a
method is provided for incorporating additives into a cross-linked poly- '
olefinic material which method comprises heating a shaped article formed
of cross linked polyolefinic material to an elevated temperature above
its crystalline melting point, contacting the article at that elevated
temperature with a bath of additive ~or a period sufficient to obtain
the desired take-up of additive per'unit area of the article surface
contacted therewith, removing the'article from contact with the additive,
maintaining the article at the elevated temperature and thereafter cool
ing the article to below the crystalline melting point to assit diffusion
of the additive within the material of the article.
The additive may be selected from one or more of antioxidants,
ageing agents, stabllizers and/or materials wh:Lch effect the surface
properties such as, for example, low friction agents.
The cross-linked polyolefin may typically be polyethylene and
the additive may be an ageing agent, an antioxidizing agent, stabilizer
and/or a low friction agent for the cross-linkecl polyethylene.
It will be appreciated that since the additive is introduced
into the material after forming has taken place the tendency for forma-
tion of blisters and the like due to gasification is very much reduced
or is substantially eliminated.
In one aspect of t~e invention, the article to be treated is a
cross-linked pipe or tube formed of polyolefin, partlcularly polyethy-
1 ene. The temperature of the polyolefin during contact with the additive
should be above the crystalline melting temperature of the cross-linked
polyolefin. The typical temperature range for polyolefin is 115C. to
180C., preferably 140C. to 160C. The article may be passed through
a bath of molten agent and the contact period with the agent may depend
on the wall thickness of the article. The fîrst stabilizing agent
employed may be one or more of the anti-oxldant commercially available
under the Trade Mark "Irganox" and the pick up of the agent is typically
within the range of 0.2% to 0.8% by weight based on the weight of cross-
linked polyole~in.
~y a variant of this aspect of the invention, the additive is
k' t ~ crc~
a low friction add:itive selected from one or more ~i~E-crystalline waxes
and clear petroleum waxes and the article is passed at elevated tempera-
ture through a bath of these materîals.
As mentioned above, typical stabilizing agents are those
commercially available under the Trade Mark "Irganox" supplied by CIBA- GEIGY
AG. One of the stabilizing agents preferred in the method of an aspect of the .
20 present invention is octadecyl 3-~3-S-ditertiary-butyl-4-hydroxyphenoI)
proprionate which is commercially available under the Trade Mark "Irganox
1076". ~nother stabilizing agent which has been successfully used in the
course of methocl o;E an aspect of the present invention is tetrakis tmeth-
ylene 3-3,5 ditertiarybutyl-4-hydroxyphenyl proprionate~ methane. This
is a very heavy molecule including four sterically hindered phenolic
.,;.
hydroxyl groups which provide high antioxidant activity. The large
molecular weight of the compound contributes to low volatility and high
extraction resistance when compounded with polymers. The material is
commercially available under the registered Trade ~lark "Irganox 1010".
In use, the oxidizing agent or mixture of oxidizing agents are
simply melted and put into a bath at the temperature of treatment. The ~;
article is then immersed for a period determined by the average wall ~
,. . . . . .
- , ~ ,~ . .. . .
0~2~3
tilickness until the pick up of material ls greater than O.~ b~ weight
based on the total weight'of the'polyolefin process. The'article is
~ then removed from the bath and is the~ maintained at the treatment
temperature for a further period of time to assist in the diffusion of
the stabili~ing agent through the'material of the article itself. For
a cross-linked polyolefin, e.g. polyethylene, the crystalline melting
point is of the order of 130C. Below 130C. the crystallites in the
cross-linked material make the diffusion or migration process of the
stabilizing agent very slow but above the crystalline melting point the
diffusion is very much faster. By introducing to the exposed outer
surface of the article a predetermined proportion of antioxldant or other
stabilizing agent and thereafter maintaining the article for a short
period of time at its contacting temperature, the material is maintained
above the crystalline melting point and fiffusion proceeds fairly rapidly.
On cooling of the article, if cooling takes place from the contacting
surface inwardly, then the distribution of the stabilizing agents through
the wall of the article is assisted. The cooling article may be
affected by rapid chilling of the external surface.
Thus, by an aspect of this invention, an article is provided
comprising a sheet of cross~linked polyolefillic material having a low '~
friction surface obtained by contacting the surface with a low friction
additive.
The invention in another aspect also includes the method of
forming a sheet of low friction material which comprises forming a tube -~
of cross~linked polyolefin materia], slitting the tube to form a longitu-
dinal sheet, passing the sheet through a bath of low friction additive
while the sheet is maintained at an elevated temperature above its
crystalline melting point, removing the sheet from the bath while main-
taining the sheet at the elevated temperature and thereafter cooling to '
below the crystalline melting point to assist diffusion of the additive
within the material of the'article.
By one variant, the additive is one or more of micro-crystalline
-- 4 --
.
' :
wax an~/or clear petroleum wax wlle~ein one surface on]y o~ the sheet is
treated.
By another variant, the wax take up is 1.4% to 2.2% by weight.
The invention in yet'a further aspect includes a ki or a boat
hull including a surface layer of low friction sheet or sheets in accor-
~ance wiLh the method as described above.
Thus~ by one variant, the sole i5 bonded is the ski using an
epoxy resin adhesive.
By another variant, the boat hull has a layer of polyolefinic
: 10 material having a low friction surface, the hull layer being formed, at
least below the water line, by one or more sheets having a low friction
surface in accordance with the method as described above
'~.
A modification of the'method of an aspect of the invention is
effected by treating the tube first with the low friction additive
together with any other additives that may be necessary or desirable and
thereafter slitting the tube longitudinally to form the sheet and opening
out the tube to form a sheet material.
bC, J, rn ~, ht
In a typical em~o~i~m~t of the present invention, the tube of
diameter such that the circumference is equivalent to the desired width ~;
of a ski sole :is extruded in the'usual way and the manner described i.n
British Patent Specification No. 1,15~,011. AEter extrusion prior to
completion of cross-linking t'he tube is slit immediately after the die.
Just before or immediately after the cross-linking of the material is
completely, the hot material is passed through a bath of molten waxes
constituting the low friction additive. The waxes are typically micro-
crystalline waxes and clear petroleum ~axes commercially available under
the Trade Mark "l~ER~". The sheet so formed is maintained in contact
with the low friction additive until the desired ta~e up of 1.2% to 2.2%
by weight of wax material has been effected. Thereafter, the cross-
linked pol~olefinic material is removed rom the contact ~ith the low
friction additive and is passed through a die or calibration tool which
~ 52~.3
. ~
permanently flattens the material to render it suitable for application ~'
to a ski sole. The materîal is then'cooled'faîrly rapidly by passage
through a water bath thereby allowing the difEusion of the low friction
additive to the material to proceed to completion.
The sheet thus formed is then attached to the sole of the ski
concerned. 'rhe surface to be secured to the ski is first ground to
increase the surface area and surface energy and is then flame treated
to oxidize the polyolefinic layer using, for instance~ propyne which
is sometimes known as "Mapgas". The'exposed layer is further treated
with a corona discharge further to oxidize the layer and then the
material, with the low friction outermost is then stuck to the ski using
an epoxy adhesive such as that commercially available under the Trade
Mark "Araldite" from Ciba-Geigy.
It has been found that skis so produced have much lower
coefficient of friction and maintain a low efficient fric~ion more or
less permanently and thus substantially avoid the need for continual
waxing of the skis for optimum performance.
The additive should be soluble in the polyolefinic material in
order to enable adequate and sufficien~ diffusion into the material ~
se to take place. - '
Following is a description by way of example on~y of methods
of carrying aspects of the invention into eEfect.
A bath was made up of 7 parts of Irganox 1076 and 25 parts of ~'
Irganox l010, the parts being by weight. A tube of 30 -mm outside diameter
and wall thickness 2.5 mm was formed of cross~linked polyethylene and was
fed direct from its extruder into a bath of material heated to 150C.
The tube was passed through the bath so as to maintain contact therewith
for a period of the order of 6 scconds. At the end of this time, the
~ take up of antioxidants into the'cross-linked polyethylene is oE the
order of 0.3% by weight based'on the weight of the polye~hylene. The tube
is then removed from the bath and passed through a heating chamber whereby
the temperature of the'tube is maintained'at or above the temperature of
contact with the antioxidants for 15 seconds. Thereafter the tube is
passed into a water bath to chill tlle tube and allow crystallization of
the material to occur.
Samples of the tube so produced were heated in an oven to
150~C. together with samples of similar tubes containing stabilizers
incorporated in the conventional manner, i~e. prior to cross-linking.
The tubes produced in accordance with the example described were with-
drawn after six weeks and did not appear to have suffered any visible
degradation. Untreated tubes lasted for one day. Tubes with conven
tional stabilization lasted five days, and up to three weeks if the
proportion of antioxidant was so high that blisters appeared in the
material of the tube.
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