Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The invention relates to a new nucleating agent
~pore regulator) for the preparation of directly
gassed thermoplastic foams.
It is known to add physically acting propellants
(i.e. propellants with a boiling temperature below
the p~ocessing temperature of the pla~tics) to
the thermoplastic material when preparing structural
foams. Nucleating agents are used as pore regulators
to control the cell structure. One of the most
frequently used nucleating agents is talc, which
is generally added to the thermoplastic material
in amounts of from 1.0 to 2.5%. The use of talc
or other non-decomposing pore regulators as so-
called "passive nucleating agents" has the disadvantage
that when waste products from the plastics processing,
the regenerated material, is reused the "passive
nucleating agent" is always recycled into the process
again, and it is very difficult to estimate the
nucleating activity of the non-decomposed pore
regulator from the regenerated material.
If solids such as talc are mixed with the plastics
in concentrations of more than about 1~, the quality
of the plastics, particularly its colour, structu~e
and gloss, are afected.
Therefore, so-called "active nucleating agents"
are becoming increasingly importantO "Active nucleating
agents" are pore regulators which decompose during
the extrusion or injection moulding process; the
regenerated material then contains no (or at least
fewer) constituents with a nucleating effect and
can be reused without any substantial loss of quality.
Particular mention should be made of systems which
cleave carbon dioxide, of which sodium bicarbonate
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or cltric acl~ esters (~E-OS 34 11 319) or the comblnatlon o~
cltric acid or sodlurn citrate wlth sodlum bicarbonate or sodlum
carbonate (Japanese Patent 715 375) have been ~escrlbed. The
citrlc acld/sodlum blcarbonate system ls effectlve as a pore
regulator when added ln amounts o~ from 0.4 to 1.0%
It has now been found that mixtures of citrlc acld,
citrlc acld esters or salts of cltrlc acld with carbonates or
bicarbonates as active nucleating agents with talc as the passlve
nucleatlng agent ln a wlde range of mixlng ratlos have better
nucleatlon properties than the actlve or passlve nucleatlng agents
on their own.
The lnventlon thus relates to an agent for nucleatlng
directly gassed thermoplastlc foams conslstlng of a mlxture of
components A, B and C, whereln
A represents monosodium citrate
and
:` B represents an alkall, alkallne earth or ammonium carbonate,
or an alkall, alkaline earth or ammonium bicarbonate
and
C represents talc, whereln components A and ~ are present in
,
a stoichlometrlc or substantlally stolchlometrlc ratlon and
. wherein components A, B and C are present ln the ratlo
. [(A + B): c] from [9:1] to [1:9]
and thls agent may, if deslred, contaln the conventlonal flow
promoting agents, release agents and lubrlcants.
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The two components A and B represent the system
described hereinbefore of an active nucleating
agent (propellant) whilst component C represents
the system of a passive nucleating agent.
The citric acid may be present in pure form or
as a hydrate and in the form of its alkali, alkaline
earth or ammonium salts. The salts of the monobasic
citric acid are preferred, such as monosodium citrate,
calcium citrate and monoammonium citrate or a 2:1
mixture of citric acid and trisodium citrate.
Particular mention should be made of monosodium
citrate.
Suitable citric acid esters are the mono- and diesters
of alcohols having up to 8 carbon atoms. Lower alcohols
are methanol, ethanol, propanol, isopropanol, n-butanol,
isobutanol, sec.-butanol, tert.-butanol, n-pentan-l-ol,
n-pentan-2-ol, n-pentan-3-ol and the isomeric pentanols,
n-hexan-l-ol, n-hexan-2-ol, n-hexan-3-ol and the isomeric
hexanols, n-heptan-l-ol, n-heptan-2-ol, n-heptan-3-ol,
n-heptan-4-ol and the isomeric heptanols, n-octan-l-ol,
n-octan-2-ol, n-octan-3-ol, n-octan-4-ol and the isomeric
octanols, cyclopentanol and cyclohexanol.
It is also possible to use esters of diols or polyols
with up to 8 carbon atoms, such as ethyleneglycol,
glycerol, pentaerythritol or lower polyethylene
glycols with up to 8 carbon atoms, e.g. diethyleneglycol,
triethyleneglycol or tetraethyleneglycol~
s
Particular mention should be made of the mono-
or diesters of monohydric alcohols with up to 6
carbon atoms.
The mono- or diesters of monohydric alcohols with
up to 4 carbon atoms are preferred.
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The monoesters such as monomethylcitrate, monoethylcitrate,
monopropylcitrate, monoisopropylcitrate, mono-n-
butylcitrate and mono-tert butylcitrate are particularly
preferred.
Monoesters according to the preceding defini~ion
include both the citric acid ~-esters and also
the citric acid ~-esters or mixtures of ~- and
~-monoesters of any desired composition. By mixing
the ~- and ~-esters in any desired proportions
it is possible to obtain mixtures with lower melting
points than the pure esters.
Diesters in accordance with the preceding definition
include both the ~,'-diesters of citric acid and
also the ~ diesters or mixtures of the a,a'-
and a,~-diesters of citric acid.
By mixing the a,~'- and a,~-diesters in suitable
proportions it is possible to obtain mixtures with
lower melting points than the pure diesters.
Diesters according to the preceding definition
also include mixed esters of two different alcohols,
and this term also includes all possible structurally-
isomeric and stereoisomeric compounds in pure form
or in any mixture.
The carbonate or bicarbonate component (B) used
may be a carbonate or bicarbonate of the alkali
metals or the alkaline earth metals or of ammonium.
Sodium, potassium and calcium are preferred.
Particular mention should be made of sodium carbonate
and sodium bicarbonate.
The citric acid component (component A) is mixed
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with the carbonate or bic~rbonate component tcomponent B)
preferably in a stoichiornetric or nearly stoichiometric
ratio, so that the free acid equivalents of component
A serve formally to neutralise component B and
as a result the optimum quantity of carbon dioxide
is released.
The standard commercial grades of talc may be used
as the talc (component C). Grades of talc which
are also used for pharmaceutical preparations are
preferred since these will be safe to eat and will
therefore not affect the quali~y of the material
in this respect. It should be particularly emphasised
that these grades of talc do not contain any mineral
fibres which could be harmful to the lungs.
The usual agents for improving flow or release
agents or lubricants may be added to the mi~ture
according to the invention.
These additives are generally known in the art.
Examples include metal-soaps such as calcium or
zinc or magnesium stearate; paraffins; silicates
such as aerosil; silicones, waxes; fats.
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Suitable thermoplastics include:
polyethylenes; ethylene-vinylacetate copolymers;
ethylene-ethylacrylates; ionomeric polyethylenes;
polypropylenes; polybutenes; polymethylpentenes;
polystyrenes; high-impact polystyrenes; styrene-
acrylonitrile copolymers; acrylic-butadiene-styrene
copolymers; ASA, polyvinylcarbazoles; polyvinylchlorides;
polytrifluoroethylenes; polytetrafluoroethylene;
perfluoropropylenes; polyvinylidene fluorides;
ethylene-tetrafluoroethylene copolymers; polymethyl-
methacrylates; polyamides; polyethyleneglycol tere-
*acrylonitrilestyryl acrylate
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phthalates; ~olybutylene terephthalates, polyoxy-
methylenes; polycarbonates; chlorinated polyethers;
phenoxy resins; polyphenylene oxides; polysulphones;
polyethersulphones; polyphenylenesulphides; polyurethane
elastomers; cellulose acetates; cellulose propionates;
cellulose-aceto-butyrates and other thermoplastics
or elastomers.
The three components A, B and C may be mixed together
as a finely ground free flowing powder. It is
also possible to coat the mixture by conventional
methods (to render it water-repellant) or to encapsulate
it, whilst all three components may be encapsulated,
coated or made water-repellant together or one
or two components are first encapsulated, coated
or made water-repellant and the remaining components
are then added.
Numerous substances are suitable for encapsulating,
coating or rendering water-repellant. Those substances
which melt or are already molten at the temperatures
conventional for thermoplastic melts but are solid
or solidify in a waxy state at ambient temperature
are preferred.
In addi~ion to the numerous waxes, fatty acids
and the esters and salts thereof are preferred
calcium stearate and magnesium stearate and the
fatty acid glycerides are particularly suitable.
The ratio of talc (C) to the,other two components
(A ~ B) can be freely selected within the limits
(C) : (A ~ B) irom 1:9 to 9:1. The range (C) :
~A ~ B) from 5:5 to 8:2 is preferred.
Particula~ mention should be made of mixtures of
monosodium citrate and sodium bicarbonate and talc
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and citric acid, sodium bicarbonate and talc.
It has proved advantageous if first of all the
sodium bicarbonate on its own and/or the citric
acid on its own is made water-repellant with calcium
stearate or fatty acid esters, because these mixtures
are particularly resistant to moisture in the air
and will not go lumpy or decompose even when s~ored
for lengthy periods.
For the citric acid esters, mixtures of monoisopropyl-
citrate with sodium hydrogen carbonate and talc
are preferred. The liquid monoisopropylcitrate
is encapsulated using methods known ~ se or may
be incorporated in plastics in the form of katches.
Exampte 1
Preparation of directly gassed polystyrene structural
foam films
The tests are carried out using a Berstoff-Tandem
apparatus. 5-7~ of a 1:1 mixture of trichloromonofluoro-
methane and dichlorodifluoromethane are used as
propellant. The thermoplastic material used is
industrial-grade polystyrene (type DOW 648).
The quantities of propellant and nucleating agent
specified relate to the quantity of polystyrene
used, and the addition of nucleating agent is varied
(whilst the addition of propellant remains constant)
until the polystyrene films have approximately
the same cell structure. The film described in
Experiment number 1 serves as the standard.
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E~periment 1 (prior art)
Polystyrene: polystyrene of the Styron 648 type
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(made by Dow Chemicals)
Nucleating agent: 1.7% tal~
Results: uniform structure, film has a pale yellow
shimmer
Experiment 2 (prior art)
Polystyrene: polystyrene of the Styron 648 type
Nucleating agent: 0.45~ of a mixture of monoscdium
citrate and sodium bicarbonate (corresponding to
Japanese Patent 715 ~375)
Results: fine uniform structure
Exp~__ment 3 (according to the invention)
Polystyrene: Polystyrene Dow 648
Nucleating agent: 0.35% talc
0.09% of a mixture of monosodium
citrate and sodium hydrogen carbonate (according
to Japanese Patent 715 375)
Results: fine uniform structure with finer pores
than in Experiment 1.
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The electron-microscope pictures enclosed as Annex 1
and 2 are sections taken longitudinally and at right
angles to the direction of extrusion of the foam film
and demonstrate the improvement in pore quality.
Examples 4 6
Direct gassing of LDPE (i.e. low density polyethylene3
Single screw extruder with gassing device, tubular
die ~conventional construction for the manufacture
o foam filmsl.
LDPE:~ density about 0.918
Melting index about 2
Gas: dichlorodifluoro ethane.
Q~antity of qas: 4 - 5~
Foam density: 0.3 - 0.4 g/ml
radem~r k
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Experiment 4: ~prior art)
LDPE: as above
Nucleating agent: 0.6 - 0.8% talc
Result: uniform foam structure, colour yellowy-grey
Experiment 5 (prior art)
LDPE: as above
Nucleating agent: 0.3 - 0.4~ analogously to Experiment 2
Result: uniform fine structure, colour lighter
than in Experiment 4
Experiment 6:
LDPE: as above
Nucleating agent: 0.25 - 0.4% analogously to Experiment 3
Result: uniform fine structure, colour lighter
than Experiment 4
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