U.S. patent application number 11/884900 was filed with the patent office on 2008-10-16 for scorch-retarding polymer composition.
Invention is credited to Jan-Ove Bostrom, Claes Broman, Ruth Dammert, Bill Gustafsson, Nigel Hampton, Lena Lindbom, Ulf Nilsson, Perry Nylander, Annika Smedberg.
Application Number | 20080254289 11/884900 |
Document ID | / |
Family ID | 34933985 |
Filed Date | 2008-10-16 |
United States Patent
Application |
20080254289 |
Kind Code |
A1 |
Bostrom; Jan-Ove ; et
al. |
October 16, 2008 |
Scorch-Retarding Polymer Composition
Abstract
The present invention relates to crosslinkable polymer
composition, comprising an unsaturated polyolefin having a total
amount of carbon-carbon double bonds/1000 carbon atoms of at least
0.1, at least one scorch retarder, and at least one crosslinking
agent.
Inventors: |
Bostrom; Jan-Ove; (Odsmal,
SE) ; Smedberg; Annika; (Myggenas, SE) ;
Dammert; Ruth; (Stenungsund, SE) ; Lindbom; Lena;
(Kungalv, SE) ; Hampton; Nigel; (Forest Park,
GA) ; Gustafsson; Bill; (Stenungsund, SE) ;
Nilsson; Ulf; (Stenungsund, SE) ; Nylander;
Perry; (Goteborg, SE) ; Broman; Claes;
(Odsmal, SE) |
Correspondence
Address: |
FAY SHARPE LLP
1100 SUPERIOR AVENUE, SEVENTH FLOOR
CLEVELAND
OH
44114
US
|
Family ID: |
34933985 |
Appl. No.: |
11/884900 |
Filed: |
February 23, 2006 |
PCT Filed: |
February 23, 2006 |
PCT NO: |
PCT/EP2006/001649 |
371 Date: |
November 20, 2007 |
Current U.S.
Class: |
428/375 ;
427/117; 524/287; 524/315; 524/343; 524/580 |
Current CPC
Class: |
C08K 5/01 20130101; C09D
123/083 20130101; Y10T 428/2933 20150115; H01B 3/441 20130101; C08K
5/14 20130101; C08K 5/01 20130101; C08L 23/02 20130101; C08K 5/14
20130101; C08L 23/02 20130101; C08K 5/14 20130101; C08L 23/083
20130101; C08K 5/01 20130101; C08L 23/083 20130101 |
Class at
Publication: |
428/375 ;
524/580; 524/287; 524/343; 524/315; 427/117 |
International
Class: |
B32B 27/32 20060101
B32B027/32; C08K 5/10 20060101 C08K005/10; C08K 5/13 20060101
C08K005/13; B05D 3/02 20060101 B05D003/02 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 28, 2005 |
EP |
05004359.5 |
Claims
1. A crosslinkable polymer composition, comprising (i) an
unsaturated polyolefin having a total amount of carbon-carbon
double bonds/1000 carbon atoms of at least 0.1, (ii) at least one
scorch retarder, and (iii) at least one crosslinking agent.
2. The polymer composition according to claim 1, wherein the
unsaturated polyolefin has a total amount of carbon-carbon double
bonds/1000 carbon atoms of at least 0.35.
3. The polymer composition according to claim 1, wherein at least
some of the carbon-carbon double bonds are vinyl groups.
4. The polymer composition according to claim 3, wherein the
unsaturated polyolefin has a total amount of vinyl groups/1000
carbon atoms of at least 0.04.
5. The polymer composition according to claim 1, wherein the
unsaturated polyolefin is prepared by copolymerizing an olefin
monomer and at least one polyunsaturated comonomer.
6. The polymer composition according to claim 5, wherein the
unsaturated polyolefin has an amount of vinyl groups/1000 carbon
atoms which originate from the polyunsaturated comonomer, of at
least 0.03.
7. The polymer composition according to claim 5, wherein at least
one polyunsaturated comonomer is a diene.
8. The polymer composition according to claim 7, wherein the diene
is selected from 1,7-octadiene, 1,9-decadiene, 1,11-dodecadiene,
1,13-tetradecadiene, 7-methyl-1,6-octadiene,
9-methyl-1,8-decadiene, or mixtures thereof.
9. The polymer composition according to claim 7, wherein the diene
is selected from siloxanes having the following formula:
CH.sub.2.dbd.CH--[Si(CH.sub.3).sub.2--O].sub.n--Si(CH.sub.3).sub.2--CH.db-
d.CH.sub.2, wherein n=1 or higher.
10. The polymer composition according to claim 1, wherein at least
one crosslinking agent is a peroxide.
11. The polymer composition according to claim 5, wherein the
olefin monomer is ethylene.
12. The polymer composition according to claim 11, wherein the
unsaturated polyethylene is produced by high pressure radical
polymerisation.
13. The polymer composition according to claim 5, wherein the
unsaturated polyolefin has an ash content of less than 0.030
wt.-%.
14. The polymer composition according to claim 1, wherein the
amount of scorch retarder is from 0.005 to 1 wt.-%, based on the
weight of the polymer composition.
15. The polymer composition according to claim 1, wherein the
scorch retarder is selected from 2,4-diphenyl-4-methyl-1-pentene,
substituted or unsubstituted diphenylethylene, quinone derivatives,
hydroquinone derivatives, monofunctional vinyl containing esters
and ethers, or mixtures thereof.
16. A crosslinked polymer composition, prepared by treatment of the
polymer composition according to claim 1 under crosslinking
conditions.
17. The crosslinked polymer composition according to claim 16,
wherein the percentage level of removable volatiles remaining in
the crosslinked polymer composition is less than or equal to 0.5 wt
% of the crosslinked polymer composition after a period of time
which is at least 10% shorter compared to the period of time which
is necessary to decrease the level of volatiles in a reference
material to the same value, wherein the reference material is a
crosslinked polymer composition prepared from an unsaturated
polyolefin having a total amount of carbon-carbon double bonds/1000
carbon atoms of 0.37 and a crosslinking agent but without a scorch
retarder.
18. The crosslinked polymer composition according to claim 16,
having a total weight change between 0-30 minutes at 175.degree. C.
of less than 1.12 wt.-% as measured according to HD632 A:1 1998,
part 2 on a plaque having a thickness of 1.8 mm.
19. A process for the preparation of a crosslinked polymer
composition, wherein the crosslinkable polymer composition
according to claim 1 is provided, followed by treatment of the
polymer composition under crosslinking conditions.
20. The process according, to claim 19, wherein the crosslinkable
polymer composition is subjected to a temperature sufficient to
effect at least partial crosslinking.
21. A multilayered article, having at least one layer comprising
the crosslinkable polymer composition according to claim 1.
22. The multilayered article according to claim 21, wherein the
article is a power cable.
23. A crosslinked multilayered article, having at least one layer
comprising the crosslinked polymer composition according to claim
16.
24. The crosslinked multilayered article according to claim 23,
wherein the article is a power cable.
25. A process for the production of a multilayered article, wherein
the crosslinkable polymer composition according to claim 1 is
applied as one or more layers onto a substrate by extrusion.
26. The process according to claim 25, wherein extrusion is
effected at a temperature satisfying the following relationship: ti
(19517/(273.15+T))-1n t.ltoreq.43.55 wherein T: extrusion
temperature in .degree. C., and t: time in minutes it takes at the
extrusion temperature T from the start of the torque measurement to
reach an increase in torque of 1 dNm from the minimum value in the
torque curve.
27. The process according to claim 25 wherein the unsaturated
polyolefin, the crosslinking agent and the scorch retarder are
blended in a single step, followed by feeding the obtained mixture
into the extruder.
28. The process according to claim 25, wherein the crosslinkable
polymer composition is prepared by blending the unsaturated
polyolefin with the scorch retarder first, followed by blending the
obtained mixture with the crosslinking agent, and feeding the final
mixture into the extruder.
29. The process according to claim 25, wherein a melt of the
unsaturated polyolefin is provided in the extruder, followed by
adding the scorch retarder and the crosslinking agent in the hopper
or to the melt, either simultaneously or in subsequent steps.
30. The process according to claim 25, wherein the multilayered
article is a power cable and the crosslinkable polymer composition
is applied onto the metallic conductor and/or at least one coating
layer thereof.
31. The process according to claim 25, wherein the crosslinkable
polymer composition is treated under crosslinking conditions.
32. The process according to claim 25, wherein after the
crosslinking step a degassing step is carried out to remove
volatile products.
33. A process for producing the crosslinkable composition of claim
1 which includes the use of a scorch retarder for extrusion of an
unsaturated polyolefin having a total amount of carbon-carbon
double bonds/1000 carbon atoms of at least 0.1.
Description
[0001] The present invention relates to polymer compositions having
low scorch during extrusion. Furthermore, it relates to articles,
in particular multilayered articles like power cables, comprising
such polymer compositions.
[0002] In general, the degree of unsaturation of polyolefins is
dependent on specific conditions chosen for the polymerisation
process. This is true for high pressure as well as low pressure
conditions. If e.g. polyethylene is produced by radical
polymerisation (so-called low-density polyethylene LDPE), the
number of double bonds within the polymer is usually quite low.
However, in many situations, it is desirable to use polymers having
a higher degree of unsaturation which may serve to introduce
functional groups into the polymer molecule or to effect
crosslinking of the polymer.
[0003] The crosslinking of polyolefins like polyethylene is
relevant for many applications, such as extrusion (e.g. of tubes,
cable insulating material or cable sheathing), blow moulding, or
rotational moulding. In particular in cable technology,
crosslinking is of special interest since deformation resistance at
elevated temperature of the cable can be improved.
[0004] In WO 93/08222, an unsaturated low-density polyethylene
(LDPE) having improved crosslinking properties was prepared by high
pressure radical polymerisation of ethylene and a specific type of
polyunsaturated comonomers. The increased amount of unsaturation of
the LDPE copolymer increases the crosslinking activity when
combined with a crosslinking agent.
[0005] As indicated above, crosslinkable polyolefins are of
interest for applying coating layers on power cables by extrusion.
In such an extrusion process of a power cable, the metallic
conductor is generally first coated with a semiconductive layer,
followed by an insulating layer and another semiconductive layer.
These layers are normally crosslinked and are normally made of
cross-linked ethylene homopolymers and/or ethylene copolymers.
[0006] Cross-linking can be effected by adding free-radical forming
agents like peroxides to the polymeric material prior to or during
extrusion. The free-radical forming agent should preferably remain
stable during extrusion performed at a temperature low enough to
minimize the early decomposition of the peroxide but high enough to
obtain proper melting and homogenisation. Furthermore, the
crosslinking agent should decompose in a subsequent cross-linking
step at elevated temperature. If e.g. a significant amount of
peroxide already decomposes in the extruder, thereby initiating
premature crosslinking, this will result in the formation of
so-called "scorch", i.e. inhomogeneity, surface unevenness and
possibly discolouration in the different layers of the resultant
cable. Thus, any significant decomposition of free-radical forming
agents during extrusion should be avoided. On the other hand,
thermal treatment at the elevated temperature of the extruded
polyolefin layer should result in high crosslinking speed and high
crosslinking efficiency.
[0007] In EP-A-0453204 and EP-A-0475561,
2,4-diphenyl-4-methyl-1-pentene is added to polymer compositions to
suppress the formation of scorch. These applications do not relate
to unsaturated polyolefins.
[0008] Furthermore, during the crosslinking step, by-products can
be generated due to decomposition of crosslinking agents. Most
by-products are captured within the cable and the volatile fraction
thereof has to be removed in a so-called degassing step. The more
by-products generated, the longer the degassing time and/or the
higher the degassing temperature. However, mild degassing
conditions would be preferred. Milder degassing conditions would
also reduce the risk of damaging the cables during the degassing
step.
[0009] One object of the present invention is to provide a
polyolefin composition having low scorch during extrusion but
improved crosslinking properties if vulcanized after extrusion. In
particular, it is an object to increase crosslinking speed and/or
to reduce the amount of crosslinking agent without adversely
affecting scorch behaviour. Furthermore, it is an object to provide
crosslinked polymer articles which can be degassed at reduced
degassing time and/or under mild degassing conditions, in
particular lower degassing temperature.
[0010] These objects are solved by the polymer compositions and the
processes as defined in the claims.
[0011] The crosslinkable polymer composition according to the
present invention comprises
(i) an unsaturated polyolefin having a total amount of
carbon-carbon double bonds/1000 carbon atoms of at least 0.1, (ii)
at least one scorch retarder, and (iii) at least one crosslinking
agent.
[0012] In the context of the present invention, the term "total
amount of carbon-carbon double bonds" refers to those double bonds
originating from vinyl groups, vinylidene groups and trans-vinylene
groups. The amount of each type of double bond is measured as
indicated in the experimental part.
[0013] The incorporation of the total amount of carbon-carbon
double bonds according to the present invention within the
polyolefin component enables to accomplish improved crosslinking
properties.
[0014] In a preferred embodiment, the total amount of carbon-carbon
double bonds is at least 0.15/1000 C-atoms. In other preferred
embodiments, the total amount of carbon-carbon double bonds is at
least 0.20, at least 0.25, at least 0.30, at least 0.35, at least
0.40, at least 0.45, at least 0.50, at least 0.55, at least 0.60,
at least 0.65, at least 0.70, at least 0.75 or at least 0.80/1000
C-atoms.
[0015] The total amount of vinyl groups is preferably at least
0.04/1000 carbon atoms. In other preferred embodiments, it is at
least 0.08, at least 0.10, at least 0.15, at least 0.20, at least
0.25, at least 0.30, at least 0.35, at least 0.40, at least 0.45,
at least 0.50, at least 0.55, at least 0.60, at least 0.65, at
least 0.70, at least 0.75 or at least 0.80 vinyl groups/1000 carbon
atoms. Of course, since a vinyl group is a specific type of
carbon-carbon double bond, the total amount of vinyl groups for a
given unsaturated polyolefin does not exceed its total amount of
double bonds.
[0016] Two types of vinyl groups can be differentiated. One type of
vinyl group is generated by the polymerisation process (e.g. via a
.beta.-scission reaction of a secondary radical) or results from
the use of chain transfer agents introducing vinyl groups. Another
type of vinyl group may originate from a polyunsaturated comonomer
used for the preparation of the unsaturated polyolefin, as will be
described later in greater detail.
[0017] Preferably, the amount of vinyl groups originating from the
polyunsaturated comonomer is at least 0.03/1000 carbon atoms. In
other preferred embodiments, the amount of vinyl groups originating
from the polyunsaturated comonomer is at 0.06, at least 0.09, at
least 0.12, at least 0.15, at least 0.18, at least 0.21, at least
0.25, at least 0.30, at least 0.35 or at least 0.40/1000 carbon
atoms.
[0018] In addition to the vinyl groups originating from the
polyunsaturated comonomer, the total amount of vinyl groups may
further comprise vinyl groups originating from a chain transfer
agent which introduces vinyl groups, such as propylene.
[0019] Preferred unsaturated polyolefins of the present invention
may have densities higher than 0.860, 0.880, 0.900, 0.910, 0.915,
0.917, or 0.920 g/cm.sup.3.
[0020] The polyolefin can be unimodal or multimodal, e.g.
bimodal.
[0021] In the present invention, the unsaturated polyolefin is
preferably an unsaturated polyethylene or an unsaturated
polypropylene. Most preferably, the unsaturated polyolefin is an
unsaturated polyethylene. Unsaturated polyethylene of low density
is preferred. In a preferred embodiment, the unsaturated
polyethylene contains at least 60 wt-% ethylene monomer units. In
other preferred embodiments, the unsaturated polyethylene contains
at least 70 wt-%, at least 80 wt-% or at least 90 wt-% ethylene
monomer units.
[0022] Preferably, the unsaturated polyolefin is prepared by
copolymerising at least one olefin monomer with at least one
polyunsaturated comonomer. In a preferred embodiment, the
polyunsaturated comonomer consists of a straight carbon chain with
at least 8 carbon atoms and at least 4 carbon atoms between the
non-conjugated double bonds, of which at least one is terminal.
[0023] Ethylene and propylene are preferred olefin monomers. Most
preferably, ethylene is used as the olefin monomer. As a comonomer,
a diene compound is preferred, e.g. 1,7-octadiene, 1,9-decadiene,
1,11-dodecadiene, 1,13-tetradecadiene, or mixtures thereof.
Furthermore, dienes like 7-methyl-1,6-octadiene,
9-methyl-1,8-decadiene, or mixtures thereof can be mentioned.
[0024] Siloxanes Having the Following Formula:
CH.sub.2.dbd.CH--[Si(CH.sub.3).sub.2--O].sub.n--Si(CH.sub.3).sub.2--CH.d-
bd.CH.sub.2,
wherein n=1 or higher can also be used as a polyunsaturated
comonomer. As an example, divinylsiloxanes, e.g.
.alpha.,.omega.-divinylsiloxane, can be mentioned.
[0025] In addition to the polyunsaturated comonomer, further
comonomers can optionally be used. Such optional comonomers are
selected from C.sub.3-C.sub.20 alpha-olefins such as propylene,
1-butene, 1-hexene and 1-nonene, polar comonomers such as
acrylates, methacrylates or acetates.
[0026] As an example, the crosslinkable polymer composition may
contain small amounts of one or more polar comonomer units, such as
1-100 micromole, 2-80 micromole and 5-60 micromole polar comonomer
units per gram of unsaturated polyolefin.
[0027] The unsaturated polyolefin can be produced by any
conventional polymerisation process. Preferably, it is produced by
radical polymerisation, such as high pressure radical
polymerisation. High pressure polymerisation can be effected in a
tubular reactor or an autoclave reactor. Preferably, it is a
tubular reactor. Further details about high pressure radical
polymerisation are given in WO93/08222, which is herewith
incorporated by reference. However, the unsaturated polyolefin can
also be prepared by other types of polymerisation process such as
coordination polymerisation, e.g. in a low pressure process using
any type of supported and non-supported polymerization catalyst. As
an example, multi-site including dual site and single site catalyst
systems such as Ziegler-Natta, chromium, metallocenes of transition
metal compounds, non-metallocenes of late transition metals, said
transition and later transition metal compounds belonging to group
3-10 of the periodic table (IUPAC 1989). The coordination
polymerization processes and the mentioned catalysts are well-known
in the field and may be commercially available or produced
according to known literature.
[0028] The crosslinkable polymer composition according to the
present invention further comprises a crosslinking agent. In the
context of the present invention, a crosslinking agent is defined
to be any compound capable to generate radicals which can initiate
a crosslinking reaction. Preferably, the crosslinking agent
contains at least one --O--O-- bond or at least one --N.dbd.N--
bond. More preferably, the cross-linking agent is a peroxide known
in the field.
[0029] The cross-linking agent, e.g. a peroxide, is preferably
added in an amount of 0.1-3.0 wt.-%, more preferably 0.15-2.6
wt.-%, most preferably 0.2-2.2 wt.-%, based on the weight of the
crosslinkable polymer composition.
[0030] As peroxides used for crosslinking, the following compounds
can be mentioned: di-tert-amylperoxide,
2,5-di(tert-butylperoxy)-2,5-dimethyl-3-hexyne,
2,5-di(tert-butylperoxy)-2,5-dimethylhexane,
tert-butylcumylper-oxide, di(tert-butyl)peroxide, dicumylperoxide,
di(tert-butylperoxy-isopropyl)benzene,
butyl-4,4-bis(tert-butylperoxy)valerate,
1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane,
tert-butylperoxybenzoate, diben-zoylperoxide.
[0031] Preferably, the peroxide is selected from
2,5-di(tert-butylperoxy)-2,5-dimethyl-hexane,
di(tert-butylperoxy-isopropyl)benzene, dicumylperoxide,
tert-butylcumylperoxide, di(tert-butyl)peroxide, or mixtures
thereof. Most preferably, the peroxide is dicumylperoxide.
[0032] The crosslinkable polymer composition according to the
present invention further comprises a scorch retarder. In the
context of the present invention, a "scorch retarder" is defined to
be a compound that reduces the formation of scorch during extrusion
of a polymer composition if compared to the same polymer
composition extruded without said compound. Besides scorch
retarding properties, the scorch retarder may simultaneously result
in further effects like boosting, i.e. enhancing crosslinking
performance.
[0033] Preferably, the scorch retarder is selected from
2,4-diphenyl-4-methyl-1-pentene, substituted or unsubstituted
diphenylethylene, quinone derivatives, hydroquinone derivatives,
monofunctional vinyl containing esters and ethers, or mixtures
thereof. More preferably, the scorch retarder is selected from
2,4-diphenyl-4-methyl-1-pentene, substituted or unsubstituted
diphenylethylene, or mixtures thereof. Most preferably, the scorch
retarder is 2,4-diphenyl-4-methyl-1-pentene.
[0034] Preferably, the amount of scorch retarder is within the
range of 0.005 to 1.0 wt.-%, more preferably within the range of
0.01 to 0.8 wt.-%, based on the weight of the crosslinkable
polyolefin composition. Further preferred ranges are 0.03 to 0.75
wt-%, 0.05 to 0.70 wt-% and 0.07 to 0.50 wt-%, based on the weight
of the crosslinkable polyolefin composition.
[0035] Since the unsaturated polyolefin component of the present
invention is provided with a total amount of carbon-carbon double
bonds/1000 C-atoms of at least 0.1, it is more reactive compared to
a material without double bonds. It could then be assumed that the
unsaturated polymeric material is more prone to scorch. However,
with the crosslinkable polyolefin composition of the present
invention, it is unexpectedly possible to maintain the good
crosslinking properties in the vulcanisation step and an improved
resistance to scorch, although the composition has an increased
reactivity.
[0036] The polymer composition may contain further additives, such
as antioxidants, stabilisers, processing aids, and/or crosslinking
boosters. As antioxidant, sterically hindered or semi-hindered
phenols, aromatic amines, aliphatic sterically hindered amines,
organic phosphates, thio compounds, and mixtures thereof, can be
mentioned. Typical cross-linking boosters may include compounds
having an allyl group, e.g. triallylcyanurate,
triallylisocyanurate, and di-, tri- or tetra-acrylates. As further
additives, flame retardant additives, acid scavengers, inorganic
fillers and voltage stabilizers can be mentioned.
[0037] If an antioxidant, optionally a mixture of two or more
antioxidants, is used, the added amount can range from 0.005 to 2.5
wt-%, based on the weight of the unsaturated polyolefin. If the
unsaturated polyolefin is an unsaturated polyethylene, the
antioxidant(s) are preferably added in an amount of 0.005 to 0.8
wt-%, more preferably 0.01 to 0.60 wt-%, even more preferably 0.05
to 0.50 wt-%, based on the weight of the unsaturated polyethylene.
If the unsaturated polyolefin is an unsaturated polypropylene, the
antioxidant(s) are preferably added in an amount of 0.005 to 2
wt-%, more preferably 0.01 to 1.5 wt-%, even more preferably 0.05
to 1 wt-%, based on the weight of the unsaturated
polypropylene.
[0038] Further additives may be present in an amount of 0.005 to 3
wt %, more preferably 0.005 to 2 wt %. Flame retardant additives
and inorganic fillers can be added in higher amounts.
[0039] From the crosslinkable polymer composition described above,
a cross-linked composition can be prepared by treatment under
crosslinking conditions, thereby increasing the crosslinking level.
Crosslinking can be effected by treatment at increased temperature,
e.g. at a temperature of at least 160.degree. C. When peroxides are
used, crosslinking is generally initiated by increasing the
temperature to the decomposition temperature of the corresponding
peroxide.
[0040] Due to the presence of a total amount of carbon-carbon
double bonds/1000 C-atoms of at least 0.1 within the unsaturated
polyolefin in combination with a scorch retarder, a lower
crosslinking temperature can be used, thereby still reaching
sufficiently high crosslinking levels. Lower crosslinking
temperature is beneficial in cases where temperature sensitive
materials are used. Furthermore, lower crosslinking temperature may
result in a lower amount of volatile by-products.
[0041] When crosslinking is initiated, in particular by peroxides,
residues are left in the crosslinked composition. To remove these
by-products, it is preferred to subject the crosslinked composition
to a so-called degassing step. Typically, degassing is effected at
elevated temperature. The less by-products have been generated in
the crosslinked composition, the milder are degassing conditions
and/or the less degassing time is needed.
[0042] Since crosslinking has been facilitated by providing an
unsaturated polyolefin having a total amount of carbon-carbon
double bonds/1000 C-atoms of at least 0.1, the amount of
crosslinking agent, which is necessary to achieve the same degree
of crosslinking, can be reduced. As a consequence, the amount of
by-products generated during crosslinking can be reduced and milder
degassing conditions can be chosen. An additional effect, due to
the reduced peroxide content, is also that a lower antioxidant
level may be used as well, still maintaining good resistance
against thermooxidative ageing.
[0043] The crosslinkable polymer composition of the present
invention does not only enable a reduction of the amount of
crosslinking agent but also results in a crosslinked composition
from which volatile by-products are removable within a
significantly shorter period of time. In particular for the
manufacturing of high quality crosslinked polymer compositions
having a low amount of detrimental volatile by-products, the
degassing time is reduced significantly. In the context of the
present invention, volatile by-products comprise any low-molecular
compounds which are captured within the polymer composition after
the crosslinking step and are removable by thermal treatment at a
temperature low enough to avoid significant degradation of the
polymeric material. These volatile products are particularly
generated during the crosslinking step.
[0044] In a preferred crosslinked polymer composition of the
present invention, the percentage level of removable volatiles
still remaining in the crosslinked polymer composition is less than
or equal to 0.5 wt % of the crosslinked polymer composition, after
a period of time which is at least 10% shorter compared to the
period of time which is necessary to decrease the level of
volatiles in a reference material to the same value, i.e. to less
than or equal to 0.5 wt % of the crosslinked polymer composition,
wherein the reference material is a crosslinked polymer composition
prepared from an unsaturated polyolefin having a total amount of
carbon-carbon double bonds/1000 carbon atoms of 0.37 and a
crosslinking agent but without a scorch retarder. The period of
time for reducing the percentage level of removable volatiles to
less than or equal to 0.5 wt % of the crosslinked polymer
composition is measured on plaques as described in the experimental
part.
[0045] The crosslinked composition preferably has a total weight
change between 0-30 minutes at 175.degree. C. of less than 1.12
wt.-% as measured according to HD632 A:1 1998, part 2, on a 1.8 mm
crosslinked plaque as described in the experimental part.
[0046] Preferably, the crosslinked polymer composition has a hot
set elongation value of less than 175%, more preferably less than
100%, even more preferably less than 90%, determined according to
IEC 60811-2-1. Hot set elongation values are related to the degree
of cross-linking. The lower the hot set elongation value, the more
crosslinked is the material.
[0047] Total weight loss after 168 h degassing at 60.degree. C.,
measured on a 1.8 mm thick plaque having an area of 6-7
cm.times.6-7 cm, should be less than 2.2 wt %, more preferably less
than 2.1 wt %, and even more preferably less than 2.0 wt % and most
preferably less than 1.9 wt %.
[0048] Weight loss and weight change data have been obtained by
measurements on plaques. All plaques used in the present invention
were made according to the same method as described in the
experimental part under "(b) Pressing of plaques for hot set, TGA
and plaque degassing measurements".
[0049] From the crosslinkable polymer composition of the present
invention, a multilayered article can be prepared wherein at least
one layer comprises said polymer composition. When crosslinking is
initiated, a crosslinked multilayered article is obtained.
Preferably, the multilayered article (either crosslinked or not) is
a power cable.
[0050] In the context of the present invention, a power cable is
defined to be a cable transferring energy operating at any voltage.
The voltage applied to the power cable can be alternating (AC),
direct (DC), or transient (impulse). In a preferred embodiment, the
multilayered article is a power cable operating at voltages higher
than 1 kV. In other preferred embodiments, the power cable prepared
according to the present invention is operating at voltages higher
than 6 kV, higher than 10 kV, higher than 33 kV, higher than 66 kV,
higher than 72 kV, or higher than 110 kV.
[0051] The multilayered article can be prepared in a process
wherein the crosslinkable composition of the present invention is
applied onto a substrate by extrusion. In such an extrusion
process, the sequence of mixing the components of the crosslinkable
composition can be varied, as explained below.
[0052] According to a preferred embodiment, the unsaturated
polyolefin is mixed with one or more antioxidants, possibly in
combination with further additives, either on solid pellets or
powder or by melt mixing, followed by forming pellets from the
melt. Subsequently, the crosslinking agent, preferably a peroxide,
and the scorch retarder are added to the pellets or powder in a
second step. Alternatively, the scorch retarder could already be
added in the first step, together with the antioxidant(s). The
final pellets are fed to the extruder, e.g. a cable extruder.
[0053] According to another preferred embodiment, instead of a
two-step process, the unsaturated polyolefin, preferably in the
form of pellets or powder, the crosslinking agent, the scorch
retarder, optionally antioxidant(s) and/or further additives, are
added to a compounding extruder, single or twin screw. Preferably,
the compounding extruder is operated under careful temperature
control.
[0054] According to another preferred embodiment, a mix of all
components, i.e. including crosslinking agent and scorch retarder,
optionally antioxidant(s) and/or further additives, are added onto
the pellets or powder made of the unsaturated polyolefin.
[0055] According to another preferred embodiment, pellets made of
the unsaturated polyolefin, optionally further containing
antioxidant(s) and additional additives, are prepared in a first
step, e.g. by melt mixing. These pellets are then fed into the
cable extruder. Subsequently, crosslinking agent and scorch
retarder are either fed in the hopper or directly into the cable
extruder. Alternatively, crosslinking agent and/or scorch retarder
are already added to the pellets before feeding these pellets into
the cable extruder.
[0056] According to another preferred embodiment, pellets made of
the unsaturated polyolefin without any additional components are
fed to the extruder. Subsequently, crosslinking agent and scorch
retarder, optionally in combination with antioxidant(s) and/or
further additives, are either fed in the hopper or directly fed
into the polymeric melt within the cable extruder. Alternatively,
at least one of these components, i.e. crosslinking agent, scorch
retarder, antioxidant, or a mixture of these components is already
added to the pellets before feeding these pellets into the cable
extruder.
[0057] According to another preferred embodiment, a highly
concentrated master batch is prepared. The master batch may also
comprise one or more antioxidants, scorch retarder and crosslinking
agent. This master batch is then added to/mixed with the
unsaturated polyolefin. Alternatively, only two of these components
are present in the starting master batch whereas the third
component (i.e. either antioxidant(s), crosslinking agent, or
scorch retarder) is added separately.
[0058] When producing a power cable by extrusion, the polymer
composition can be applied onto the metallic conductor and/or at
least one coating layer thereof, e.g. a semiconductive layer or
insulating layer. Typical extrusion conditions are mentioned in WO
93/08222.
[0059] According to a preferred embodiment, extrusion, e.g. cable
extrusion, is effected at a temperature satisfying the following
relationship:
(19517/(273.15+T))-1n t.ltoreq.43.55
wherein T: extrusion temperature in .degree. C., and t: time in
minutes it takes at the extrusion temperature T from the start of
the torque measurement to reach an increase in torque of 1 dNm from
the minimum value in the torque curve.
[0060] Even more preferably, extrusion (e.g. cable extrusion) is
effected at a temperature satisfying the following
relationship:
(19517/(273.15+T))-1n t.ltoreq.43.4
wherein T and t are defined as described above.
[0061] The increase in torque is measured on a Monsanto MDR 2000
rheometer using press-moulded circular plaques as described in the
experimental part under "(c) Monsanto scorch test".
[0062] Preferably, the extrusion temperature is higher than
120.degree. C. In case extrusion is carried out in a cable
extruder, the extrusion temperature is preferably within the range
of 120.degree. C. to 160.degree. C.
[0063] In an extrusion process satisfying the relationship given
above and using the crosslinkable polymer composition according to
the present invention, an improved balance between scorch and
extrusion rate is obtained.
[0064] To produce the final power cable, the extruded polymer
composition is treated under cross-linking conditions, also known
as vulcanisation. Preferably, it is treated at a temperature of at
least 160.degree. C., even more preferably at least 170.degree. C.
When a peroxide is used, the temperature is preferably raised above
its decomposition temperature.
[0065] Due to the presence of a total amount of at least 0.1
carbon-carbon double bonds within the unsaturated polyolefin in
combination with a scorch retarder, a lower crosslinking
temperature can be used, and still reaching sufficiently high
crosslinking levels. Lower crosslinking temperature is beneficial
in cases where temperature sensitive materials are used.
[0066] Due to the improved crosslinking ability, a lower
temperature setting can be used in the continuous vulcanisation
(CV) tube. This might be relevant if strippable outer
semiconductive screens are used as they are more temperature
sensitive resulting in a lower throughput on the line. Thus, with
the crosslinkable polymer composition of the present invention, it
is possible to maintain the output even if the temperature settings
are lowered in the tube for continuous vulcanisation.
[0067] As already indicated above, the crosslinking step can result
in the formation of residues which are left in the cable
insulation. If crosslinking is initiated by peroxides, e.g.
dicumylperoxide, these by-products typically comprise compounds
like methane, ethane, cumylalcohol, .alpha.-methylstyrene or
acetophenone which are captured within the cable. To remove
volatile by-products, the cable is preferably subjected to a
degassing step.
[0068] The invention is now further elucidated by making reference
to the following examples.
EXAMPLES
Testing Methods/Measuring Methods
(a) Determination of the Amount of Double Bonds
[0069] The procedure for the determination of the amount of double
bonds/1000 C-atoms is based upon the ASTM D3124-72 method. In that
method, a detailed description for the determination of vinylidene
groups/1000 C-atoms is given based on 2,3-dimethyl-1,3-butadiene.
This sample preparation procedure has also been applied for the
determination of vinyl groups/1000 C-atoms, vinylidene groups/1000
C-atoms and trans-vinylene groups/1000 C-atoms in the present
invention. However, for the determination of the extinction
coefficient for these three types of double bonds, the following
three compounds have been used: 1-decene for vinyl,
2-methyl-1-heptene for vinylidene and trans-4-decene for
trans-vinylene, and the procedure as described in ASTM-D3124
section 9 was followed.
[0070] The total amount of double bonds was analysed by means of IR
spectrometry and given as the amount of vinyl bonds, vinylidene
bonds and trans-vinylene bonds, respectively.
[0071] Thin films were pressed with a thickness of 0.5-1.0 mm. The
actual thickness was measured. FT-IR analysis was performed on a
Perkin Elmer 2000. Four scans were recorded with a resolution of 4
cm.sup.-1.
[0072] A base line was drawn from 980 cm.sup.-1 to around 840
cm.sup.-1. The peak heights were determined at around 888 cm.sup.-1
for vinylidene, around 910 cm.sup.-1 for vinyl and around 965
cm.sup.-1 for trans-vinylene. The amount of double bonds/1000
carbon atoms was calculated using the following formulas (ASTM
D3124-72):
vinylidene/1000 C-atoms=(14.times.A)/(18.24.times.L.times.D)
vinyl/1000 C-atoms=(14.times.A)/(13.13.times.L.times.D)
trans-vinylene/1000
C-atoms=(14.times.A)/(15.14.times.L.times.D)
wherein A: absorbance (peak height) L: film thickness in mm D:
density of the material
(b) Pressing of Plaques for Hot Set, TGA and Plaque Degassing
Measurements
[0073] First, the pellets were melted at 115.degree. C. at around
20 bar for 2 minutes. The pressure was increased to 200 bar,
followed by ramping the temperature up to 165.degree. C. The
material was kept at 165.degree. C. for 25 minutes and after that
it was cooled down to room temperature at a cooling rate of
15.degree. C./min. The thickness of the plaque was around 1.8
mm.
1. Degassing Experiments as Measured by TGA on Crosslinked
Plaques
[0074] The crosslinking by-product concentration was determined
according to HD632 A1:1998, Part 2. A detailed description can be
found under 2.4.15.
[0075] The following three properties were determined: [0076] Total
weight change of the test samples during the first 30 minutes of
the test. According to the specification, this should be less than
1.6% of the original sample weight. [0077] Rate of change of weight
during the first 5 minutes of the test. [0078] Average rate of
change of sample weight between 15 to 30 minutes testing time.
[0079] These measurements were performed in a thermogravimetric
analyser (TGA). A sample taken from the crosslinked plaque directly
after its preparation, having a weight of 20.+-.5 mg is analysed.
The temperature is raised from 30.degree. C. to 175.+-.3.degree. C.
with a heating rate of 50.degree. C./min. The weight loss
experiments are carried out at a constant temperature of
175.degree. C.
2. Degassing Experiments as Measured as Total Weight Loss on
Plaques:
[0080] Directly after the pressing step, the plaque was divided
into smaller pieces with an area of 6-7 cm.times.6-7 cm. These
smaller plaques were weighed and then placed in an oven at
60.degree. C. After that, the plaques were weighed after different
periods of time. The total weight loss of the plaque after one week
of degassing (168 h) was determined.
(c) Monsanto Scorch Test.
[0081] The resistance to scorch formation of the different
formulations was evaluated in a Monsanto MDR2000 rheometer. The
experiments were carried out using press-moulded circular plaques.
The circular plaque was pressed at 120.degree. C., 2 min. without
pressure followed by 2 min. at a pressure of 5 tons. Then, the
plaque was cooled to room temperature. The increase in torque was
monitored as a function of time in the Monsanto rheometer. The time
needed to reach a certain increase of torque was determined. In
this case, the time it takes from the start of the test until an
increase of 1 dNm in torque from the minimum value in the torque
curve has been reached is reported. The longer time it takes, the
more resistant is the formulation to the formation of scorch. Data
were generated at three different temperatures, i.e. 135.degree.
C., 140.degree. C. and 145.degree. C. Data for the inventive
formulations as well as for the reference/comparative formulations
are presented in Tables 3 and 4.
(d) Elastograph Measurements of the Degree of Crosslinking
[0082] The degree of crosslinking was determined on a Gottfert
Elastograph. The measurements were carried out using press-moulded
circular plaques. First, a circular plaque was pressed at
120.degree. C., 2 min. without pressure, followed by 2 min. at 5
tons. Then, the circular plaque was cooled to room temperature. In
the Elastograph, the evolution of the torque is measured as a
function of crosslinking time at 180.degree. C. The test was used
to monitor that the degree of crosslinking was comparable in the
different samples.
[0083] The reported torque values are those reached after 10
minutes of crosslinking at 180.degree. C.
(e) Hot Set Measurements
[0084] The hot set elongation as well as the permanent deformation
were determined on samples taken from the crosslinked plaques,
prepared as described above (i.e. under (b), Pressing of plaques
for hot set, TGA and plaque degassing measurements). These
properties were determined according to IEC 60811-2-1. In the hot
set test, a dumbbell of the tested material is equipped with a
weight corresponding to 20 N/cm.sup.2. This specimen is put into an
oven at 200.degree. C. and after 15 minutes, the elongation is
measured. Subsequently, the weight is removed and the sample is
allowed to relax for 5 minutes. Then, the sample is taken out from
the oven and is cooled down to room temperature. The permanent
deformation is determined.
(f) Ash Content
[0085] Around 4 g of the polymer, exact weight is noted, was put
into a porcelain crucible. This was then heated and the sample
turns into ashes. The porcelain crucible is put into an oven at
450.degree. C. for 1 h. After that treatment, the porcelain
crucible containing the ash is allowed to cool down in a
desiccator. The ash is weighed and the ash content is
calculated.
Materials
Polymer 1:
[0086] Poly(ethylene-co-1,7-octadiene) polymer, ash
content<0.025%, MFR.sub.2=2.7 g/10 min.
Polymer 2:
[0087] Poly(ethylene-co-1,7-octadiene) polymer, ash
content<0.025%, MFR.sub.2=2.1 g/10 min.
Polymer 3:
[0088] Low density polyethylene, MFR.sub.2=2.0 g/10 min.
[0089] The double bond content of polymers 1-3 is summarised in
table 1.
TABLE-US-00001 TABLE 1 Double bond content vinyls vinyl- trans.-
total double originating vinyl/ idene/ vinylene/ bond content/ from
diene/ sample 1000 C 1000 C 1000 C 1000 C 1000 C polymer 1 0.82
0.24 0.11 1.17 0.71 polymer 2 0.26 0.21 0.06 0.53 0.15 polymer 3
0.11 0.22 0.04 0.37 --
[0090] The amount of vinyl groups originating from the
polyunsaturated comonomer (i.e. in this example 1,7-octadiene) per
1000 carbon atoms was determined as follows:
[0091] Inventive polymers 1 and 2 and reference polymer 3 have been
produced on the same reactor, basically using the same conditions,
i.e. similar temperature, pressure and production rate. The total
amount of vinyl groups of each polymer was determined by FT-IR
measurements, as described above. Then, it is assumed that the base
level of vinyl groups, i.e. the ones formed by the process without
the addition of chain transfer agent resulting in vinyl groups and
without the presence of a polyunsaturated comonomer, is the same
for the reference and for polymers 1 and 2. This base level is then
subtracted from the measured amount of vinyl groups in polymers 1
and 2, thereby resulting in the amount of vinyl groups/1000
C-atoms, which result from the polyunsaturated comonomer.
[0092] All polymers were polymerised in a high pressure tubular
reactor at a pressure of 1000 to 3000 bar and a temperature of 100
to 300.degree. C. All polymers have a density within the range of
0.920-0.925 g/cm.sup.3.
[0093] The final formulations including polymers 1-3 are summarised
in table 2.
TABLE-US-00002 TABLE 2 Summary of used formulations cross- anti-
linking scorch Elasto- oxidant agent retarder graph hot set content
content content value elonga- formulation polymer (wt.-%) (wt.-%)
(wt.-%) (Nm) tion (%) formulation 1 1 0.09 1.0 0.15 0.60 50
formulation 2 1 0.09 1.35 0.45 0.61 44 formulation 3 1 0.21 1.3
0.15 0.60 50 formulation 4 1 0.21 1.60 0.45 0.65 43 formulation 5 2
0.10 1.45 0.15 0.62 62 formulation 6 2 0.10 1.50 0.45 0.62 60
formulation 7 2 0.23 1.70 0.15 0.62 66 formulation 8 2 0.23 1.75
0.45 0.62 55 reference 3 0.20 2.1 0 0.64 56 comparative 1 0.09 0.90
0 0.61 51 formulation 1 comparative 1 0.21 1.20 0 0.60 49
formulation 2 comparative 2 0.10 1.60 0 0.60 72 formulation 3
comparative 2 0.23 1.80 0 0.63 63 formulation 4
[0094] For the formulations summarized in Table 2, the following
compounds were used as an antioxidant, a crosslinking agent and a
scorch retarder, respectively:
antioxidant: 4,4'-thiobis(2-tertbutyl-5-methylphenol) (CAS number
96-69-5) crosslinking agent: Dicumylperoxide (CAS number 80-43-3)
scorch retarder: 2,4-diphenyl-4-methyl-1-pentene (CAS number
6362-80-7)
[0095] The formulations have been crosslinked to a degree within
the range of 40-70%, as measured by the hot set test, to make the
comparison of scorch as clear as possible.
Summary of Results
Example 1
[0096] In example 1, the inventive formulations 5-6 are compared
with the reference material and comparative formulation 3 with
respect to suppression of scorch formation. The results are
presented in table 3.
TABLE-US-00003 TABLE 3 Scorch performance formulations 5 and 6
temperature that could be temperature that could be hot set time to
reach 1 dNm time to reach 1 dNm time to reach 1 dNm used if scorch
time used if scorch time elongation increase in torque increase in
torque increase in torque of ref. at 135.degree. C. of ref. at
135.degree. C. sample value (%) at 135.degree. C. (min) at
140.degree. C. (min) at 145.degree. C. (min) were allowed were
allowed in % formulation 62 110 60 35 139.1.degree. C..sup.(1)
3.03% 5 formulation 60 146 77 45 140.2.degree. C..sup.(1) 3.85% 6
ref. 56 72 43 25 135.degree. C. 0% comp. 72 69 40 25 formulation 3
.sup.(1)calculated via the exponential curve obtained from the
scorch time measured at different temperatures.
[0097] These data clearly indicate the significant difference in
the prevention of scorch that can be achieved in the unsaturated
polymers in combination with a scorch retarder like
2,4-diphenyl-4-methyl-1-pentene. Even if the same peroxide loading
can be used in comparative formulation 3 to reach the same degree
of crosslinking, the scorch time is rather short, compared to
formulations 5 and 6 according to the present invention.
[0098] Either the improved resistance to scorch formation can be
used to increase the running time until a certain torque is
reached, or the extruder could be operated at a higher melt
temperature and still have the same scorch formation as with
conventional compounds. The higher melt temperature is later on
improving the crosslinking performance as there will be a smaller
difference between the melt temperature in the extruder and the
crosslinking temperature in the vulcanising tube. Both these two
options are of advantage for the cable manufacturer as both options
increase the productivity. The first option increases productivity
as it enables longer running times before the extruder has to be
stopped and cleaned (minimizes the stop time when the extruder is
not in use) and the second option increases the productivity as the
delta in temperature, i.e. from the extruder melt temperature to
the actual crosslinking temperature, is smaller thereby increasing
the output rate of the cable line. This second option offers even
more benefits if a temperature sensitive outer semiconductive
material is used.
Example 2
[0099] Again, scorch performance of different samples was tested.
The results are shown in table 4.
TABLE-US-00004 TABLE 4 Scorch performance for inventive formulation
1 and 2 temperature that could be temperature that could be hot set
time to reach 1 dNm time to reach 1 dNm time to reach 1 dNm used if
scorch time used if scorch time elongation increase in torque
increase in torque increase in torque of ref. at 135.degree. C. of
ref. at 135.degree. C. sample value (%) at 135.degree. C. (min) at
140.degree. C. (min) at 145.degree. C. (min) were allowed were
allowed (in %) formulation 50 172.1 89.6 51.7 142.5.degree.
C..sup.(1) 5.56% 1 formulation 44 171.5 88.5 50.5 140.2.degree.
C..sup.(1) 3.85% 2 ref. 56 72 43 25 135.degree. C. 0% comp. 51
110.5 65.6 38.0 formulation 1 .sup.(1)calculated via the
exponential curve obtained from the scorch times measured at
different temperatures.
[0100] Again, the data clearly indicate an improvement of scorch
performance when a composition according to the present invention
is used, even if the unsaturated polyolefin contains a high amount
of double bonds.
Example 3
[0101] In this example, the total weight loss was measured as
indicated above. The results are summarised in table 5.
[0102] Furthermore, Table 5 provides data for the time which is
necessary to obtain a by-product level of 0.5 wt % when the polymer
compositions are subjected to a thermal treatment at 60.degree. C.
The values shown in the table were obtained as follows: The samples
were stored at 60.degree. C. and the weight was measured
continuously until there was no further measurable change of weight
between two consecutive measurements. Normally, these conditions
were obtained after 168 h degassing time. This weight of the
specimen was used as the base line for the calculations of the 0.5
wt % level of by-products. The period of time for the reference
material to reach a residual by-product level of 0.5 wt % was 19 h.
These measurements were carried out on a crosslinked plaque with an
area of 6-7 cm.times.6-7 cm and around 1.8 mm thick.
TABLE-US-00005 TABLE 5 Total weight loss and time for removal of
by-products time to reach 0.5 time to reach 0.5 wt % by-product wt
% by-product total weight loss level when treated level when
treated sample after 168 h (%) at 60.degree. C. (h) at 60.degree.
C. in (%) inventive 0.94 11 57% formulation 1 inventive 1.33 15 79%
formulation 2 inventive 1.35 16 84% formulation 3 inventive 1.54 17
89% formulation 4 inventive 1.37 15 79% formulation 5 inventive
1.50 17 89% formulation 6 inventive 1.67 formulation 7 inventive
1.77 formulation 8 reference 1.94 19 100%
[0103] If a lower peroxide loading can be used to result in the
same crosslinking 5 degree, this will also have a beneficial impact
on degassing behaviour since less by-products have to be removed
during the degassing step. This effect is exemplified by the
results shown in table 5.
[0104] Furthermore, the results of table 5 clearly demonstrate that
volatile by-products can be removed from the inventive formulations
much faster. Although all materials have a similar degree of
crosslinking as indicated by the hot set values, the inventive
formulations result in polymeric networks from which volatile
by-products can be removed faster. Thus, high quality cables having
a low amount of detrimental volatile by-products within the cable
layers can be obtained at higher production rate.
Example 4
[0105] In this example, the weight loss rate changes have been
determined as indicated above. The results are shown in table
6.
TABLE-US-00006 TABLE 6 Summary of weight rate changes Total weight
change Rate of weight change Sample (0-30 min.) (in %) (0-5 min.)
(in %/min) Inventive formulation 1 0.550 0.085 Inventive
formulation 2 0.774 0.108 Inventive formulation 3 0.740 0.110
Inventive formulation 4 0.883 0.153 Inventive formulation 5 0.755
0.136 Inventive formulation 6 0.923 0.160 Inventive formulation 7
0.868 0.132 Inventive formulation 8 0.982 0.149 reference 1.116
0.181
[0106] All formulations according to the present invention show a
lower rate of weight change than the reference which indicates that
less material is to be removed by degassing.
* * * * *