Note: Descriptions are shown in the official language in which they were submitted.
1038990
This invention relates to novel solutions for the
production of rubber articles by dipping, and to the production
~ of such solutions.
; Dipped rubber articles, such as rubber gloves, teats,
nipples, and toy balloons, have for many years been produced
from solutions of natural rubber in petrol, or from natural
latex or latexlike emulsions of natural rubber, as well as from -~
vulcanizable synthetic rubber solutions and emulsions.
In order to obtain the desired properties as regards
; lO strength, elasticity, recovery from stretching etcetera, the
- dipped articles have had to be subjected to a subsequent
- vulcanization process, i.e. a cross-linking process. Origi-
nally, a cold vulcanization was carried out by treating the
dipped articles with chlorinated sulphur, but this cold vulcan-
ization was glven up long ago in favour of hot Yulcanization,
- the vulcanizing agents and accelerators wholly or partly being
added to the rubber mixture before dissolving the latter.
Since the vulcanization step adds substantially to
the manufacturing costs, the present invent;on aims at pro-
- 20 viding so1utions preferably in aliphatic or mainly aliphatic
-; hydrocarbons for producing articles of the said kind by
dipping, in which production no vulcanization is necessary.
With this ob3ect in view, a solution for producing
rubber articles by dipping according to the present invent;on
comprises an elastomer of the synthetic block-copolymer type,
which is made sufficiently soluble ;n aliphatic hydrocarbons by
extension with from 5 to 100 percent by weight of a s~itable
~ oi1-extender, the said oil-extended elastomer being dissolved
- in 200 to 1200 percent by we;ght of a solvent consist;ng or -~
~ 30 mainly cons;sting of al;phat;c hydrocarbons.
r~ Generally, the solut;ons of the invention also `~
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1.038990
contain antioxidants, and they may contain the usual adjuvants,
including waxes for ozone protection, pigments, and fillers.
At present, the preferred elastomer is a block-
copoly~er of the styrene~butadiene type, although other syn-
thetic block-polymer types may be suitable, for instance a
block-copolymer of the styrene-isoprene type.
More particularly, the preferred elastomers are of
the kind of block-copolymers known as thermoplastic rubbers,
which do not need vulcanization, but nevertheless behave almost
as vulcanized elastomers, exhibiting high resilience and
elongation, combined with a low set at normal temperatures.
The use of mainly aliphatic hydrocarbons, as for
example petrol, is decisively advantageous both for economic ~ ~ -
reasons and because their vapours are of low toxic;ty. More-
over, solvents of this kind are generally used ;n rubber
factories for dissolving natural rubber and the non-oil
resistant synthetic rubbers, which means that no change of
. .,
solvent would be necessary in processing the present elastomers
alternating with the processing of natural rubber or the said
synthetic rubbers.
The oil extenders of the k;nd used ;n the manufacture
of natural or synthetic rubber art;cles are not all equally
well suited for use in the present solutions. The most widely
'- used extenders for rubber are petroleum oils containing pre-
dominantly aliphatic, naphthenic, and aromatic hydrocarbons,
; and of these the naphthenic and the aromatic types are particu-
larly su;table for use in the present solutions, because they
cooperate with the aliphatic hydrocarbon solvent in dissolving
the thermoplastic rubber and thus pro~ide suitable concen-
trations of the latter for the dipping process. The preferred
type is naphthenic process oil, because it is li~ht coloured
B~ 2 -
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1~8990
- and - more ;mportant - has a moderate effect only on the
- strength of the deposited film.
It has furthermore been found that the said naph-
thenic process oil extenders improve the visco-elastic proper-
ties of the solutions to a surprising degree and reduce the
shrinkage of the articles when solidifying on the dipping ;
moulds, as well as reducing the tension ;n the deposited film. ~ -
Accordingly, stripping of the articles from the moulds is easy.
Particularly good results have been obtained by using
15 to 35 percent of the naphthenic oil extender, calculated
upon the synthetic rubber weight, both as to the visco-elast;c
propert;es and as to the strength and ageing stability of the
- deposited film.
A disadvantage of the present solutions, as compared
w;th solutions of natural rubber or of the synthetic rubbers
hitherto used, is that the viscosity of the former has an :~
increased sensitivity to temperature variations at normal
temperatures. This disadvantage can be overcome by strictly
- controlling the temperature of the solutions during the
dipping. Thus, if the variations in the temperature of the
; solutions during the dipping are kept within 1C, reasonably ~ -
good results are obtained.
~; However, a manner of overcoming this disadvantage of
increased temperature sensitivity has been found, which has
proved surprisingly effective.
~, Thus, in a preferred embodiment of the present
invention, the solutions also contain a small proportion of a
dithiocarbamate, preferably zinc dibutyld;th;ocarbamate, or an
antioxydant, such as 1,3,5-trimethyl-2,4,6-tr;s~3,5-di-tert- `
3~ butyl-4-hydroxybenzyl)-benzene, being soluble ;n al;phatic
hydrocarbon solvents, for example ~.1-3.~ parts by weight per '- '
_ 3 _
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~0 ~ 9 9 0
~ 100 parts by weight of thermoplastic rubber.
- The use of dithiocarbamates as strong vulcanization
accelerators and as antioxidants is well-known in the rubber
industry, but to my best knowledge no one before has noticed - -
their extraordinary effect in stabilizing the viscosity of
synthetic rubber solutions.
Only very few vulcanization accelerators are soluble
in aliphatic hydrocarbon solvents, but two other strong
accelerators soluble in petrol, namely benzothiazolyl-2-
10 diethylsulfenamide and a condensation product of acroleines -
with aromatic bases, have been tried.
As these chemicals show very little, if any, stabi-
, lizing effect on the viscosity of the solutions, I believe that
the stabilizing effect is specific for the first mentioned
soluble accelerators/antioxidants in the present solution of
thermoplastic elastomers.
A further reduction of the viscosity of the presentsolutions can be obtained by admixing pure anhydrous ethanol or
a technical grade of anhydrous ethanol, preferably in an amount
~ 20 of 5 to 10 percent by weight as calculated upon the amount of
: synthetic rubber in the solutions.
An example of a solution according to the present
invention is as follows:
Parts by weight ~
Thermoplastic styrene-butadiene
copolymer 100
Naphthenic process oil (rubber
process oil) 20
Zinc dibutyldithiocarbamate 0.5
Extraction grade petrol (b.p. 100-130C ~50
The viscosity of this solution as determined by means
of the 8rookfield viscometer with rotor ~o. 3 at 22C ~ 1~ is
B ~ _ 4 _
about 4000 cp + 500 cp, corresponding to the viscosity of a
natural rubber solution, which is well suited for producing
dipped articles of rubber.
The accompanying drawing illustrates the effect of
the addition of extender and of dithiocarbamate at different
ratios between the thermoplastic copolymer and the solvent in
the solutions of the invention.
The curves marked A represent a ratio of 1 part of
the copolymer to 7.5 parts of solvent; that marked B represents
a ratio of 1:8.5, and those marked C represent a ratio of
1:9.5, whereas that marked D represents a natural rubber
solution in the ratio 1:8.5.
The fulldrawn curves represent solutions without
added extender or dithiocarbamate. Curves drawn with broken
lines represent solutions with added oil extender, and the
dash-dotted curves represent solutions with admixed oil -
extender and dithiocarbamate.
It will be noted that addition of the oil extender
and the dithiocarbamate both have a stabilizing effect on the
viscosity as regards variations in temperature of the
solutions.
Accordingly, the present invention also comprises a
method of producing dipped articles from solùtions of thermo-
plastic copolymers of the synthetic rubber type in aliphatic
hydrocarbon solvents, in which a dipping solution is used, the
sensitivity of which to temperature variations has been reduced
by means of the addition of an oil extender and a vulcanization
accelerator, preferably zinc dibutyldithiocarbamate, and/or an
antioxidant, preferably 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-
3~ butyl-4-hydroxybenzyl)-benzene, being solu~le in aliphatic
hydrocarbon solvents.
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