CA2056918A1 - Composite foams of low thermal conductivity - Google Patents

Abstract

0. 2 . 0050/4204g Abstract of the Disclosure: A. composite foam of low thermal conductivity comprises a) 20 80% by volume of silica aerogel particles having a mean diameter of from 0.1 to 20 mm and a density of from 0 . 08 to 0 .40 g/cm3, b) 20 - 80% by volume of a styrene polymer foam which surrounds the particles of component a) and binds them to one another and has a density of from 0 . 01 to 0.15 g/cm3, and, if desired, c) conventional additives in effective amount.

Description

20~6918 O.Z. 0~0/4~049 Compo~ita_fo~ms of low thermal conductivity ~he present invention relate~ to novel compo~ite foam~ which compri~e ~ilica aerogel particles and styrene polymer foam~ and are di~tinguiYhed by low thermal conductivity.
Convantional insulating material~ ba~ed on polyolefin~ and polyurethane~ are generally produced using organic blowing agent3, such as chlorofluoro-carbon~. The blowing agent included in the calls of the foam i3 xesponsibl~ for the high tharmal in ~lation capacity. Blowing agent~ of thi~ type are environm~ntal pollutant~ ~ince they 510wly Pscapa into the atmo~phere.
It is furthermora known that silica aerogels have an excellent th~rmal in~ulation capa ity. However, the product cannot be produced in a~y de~ired shapQ. It furthermore ha~ only low compressive strength and i~
susceptible to frac~uring. In addition, a loose aerogel pilè tends ~o ~hrink somewhat in volume due to gradual po~t-compreesion.
EP-A 340 707 proposes binding ~ilica a~rogel particle~ to ~orm compres~ion-resistant insulating material~ by means of an inorganic or organic binder.
However, the thermal conductivi~y of the~ product~ i8 unsa~i~factory.
It i3 an ob~ect of the pre~ent invention to d~velop ~n~ulating material3 havin~ a high thermal insulation capacity which are free fro~ environmentally unacceptable organic blowing agents.
It i8 a further ob~ect of the pre~ent inqention, in particular, to produce in3ulating materials of any desired shaps starting fxom ~ilica aerogel and at the ~ame time ~o compensate for the low compressive ~tr~ngth and high ~u~ceptibi}ity toward frac~ure of the aerogel and tha ~hrinkags in volume of an aerogel pile by binding thi~ substance into a matrix, while retaining the low thermal conductivity.

2~6~1~
- 2 - O . Z . 0050/420d~9 We have found that this ob~ect i~ achieved by a com3E?o8ite foam compri~ing ~ilica aerogel particles and styrene polymer foam~.
The invention accordingly provide~ a compo~ite foam of low thennal conducti~rity, compri~ing a) 20 - 80~6 by volume of ~ilica aerogel particle~
hav~ ng a mean diameter of from 0 .1 to 20 mm and density of from 0.08 to 0.40 g/cm3, b) 20 - 80% by volume of a styrene polymer foam whioh surrounds the particle~ of component a) and binds them to one another and h~ a den~ity o~ fro3r 0 . 01 to O .15 g/cm3~ and, if dehired, c: ) conventional additive~ in ef fective amount~ ~
The presant invention furthermore provide~ a.
process for the production of composite foams of thi~
type which compri~es heating an intimate mixtuxe of ~ilica aerogel particles and ~tyrene polymer foam par-ticles to a temperature above the so~tening point o~ th~3 ~tyrene pol~ner in a mold which clo~ not sQal in a ga~-tigh~ manner.
The e~ential constituent of the novel composite foam compri~e~ silica aerogel particles which hava a mean diameter o from 0.1 to 20 mm, preferably from 0.5 to 5 mm, in par~icular from 1 to 4 ~m.
The ~ilica aerogel particle~ are generally in the form of bead~ or ~ph~re and have a den~ity of from 0.05 to 0.40 g/cm3~ preferably from 0.08 to 0.35 q/cm3~ and a bulk density of from 0.04 to 0.25 gic~3 . Their thermal conductivity ~ i~ from 0.020 to ~.025 tW/m R].
The ~ilica aerogQl particles 9~8~ntially compri~e amo~phou~ S1O2 and contain traces of water and possibly small amounts of organic compounds (up to 10%), depending on the way in which they sre produced. They are produced in a conventional manner ~rom a water-glass ~olution via a sillca hydrogel by solven~ exchange and sub~equent drying. Thc bead form is produced by praying a rapidly gelling silicic acid 301 from a specially de~igned nozzle 2~6~8 - 3 - O.Z. 0~50/42049 and gelling the drops in ~he air. Further d~tail~ on thi~
proce~3 are given in D~-A 21 03 243. Replacement of the hydrogel wa~er by other liquid~ which are chemically inert toward SiO2 is de~cribed, for example, in VS-~
2,093,454, US-A 3,977,993 and JA-A 53/025,295, and the drying of the lyogels i~ described, for example, in ~S-A
2,093,454, US-A 2,249,767, FR-A 130 417, US-A 3,672,833, EP-A 0 018 955, US-A 4,327,065, EP-.A 0 067 741, DE-A
34 29 b71, EP A 0 186 149 and US-A 4,610,863. The gal liquid used for sup~rcritical d~ying i~ advantageou~ly dry mathanol, which mean~ that the ra~ultant aerogels are hydxophobic with an organic carbon content of about S~.
In the novel composite foams, the ~ilica aerogel particle~ are bound to one Another by a styrene polymer foam which ha~ 2 den~ity o~ from 0.01 to 0.15 g/cm3~
preferably from 0.015 to 0.09 g/cm3~ in particular from 0~02 to 0.07 g/~3.
~or the purpo~o3 of the pre~en~ invention, ~tyrene polymsr~ are poly~tyrene and copol~me.r~ o~
~tyrene which contain at least 50% by weight, preferab:ly at 18a8t 80% by wsight, of copolymerized styrene.
Examples of suitable comonomer~ are a-methyl~tyrene, ring-halogenated styrene~, ring-al~ylated ~tyrenes, acrylonitrile, e~ter~ of (meth)acrylic acid with alcohols having from 1 to 8 carbon atoms~ N-vinyl compounds~ such as ~inylcarbazole, maleic anhydride or alternatively sma}l ~mou~t~ of compound~ which contain two poly-merizable doubl~ bond~, such as bu~adiene, divinylbenzene or hutanediul diacrylate.
~he novel cempo~ite foam~ may furthermore contain con~entional additiva~ i~ effective amounts, such as dye3, pigment~, filler~, flameproofing agent~, ~ynergist~
for flameproofing agent~, antistatic~, stabilizers, lubrican~s, opacifier~ and the like. ~he addLtive~ may be both in the aerogel pha~e and in the foam phase.
The compo~ite foams are e~pediently produced by hea~iny an intimate mixture of silica aerogel particles 2~6318 - 4 - O.Z. ~05~/42049 and styrene polymer foam particle~ to a temperature above tha 90ftening point of the styrene pulymer in a mold which doe~ not se~l in a ga~-tight manner. Foam particl3s having a particle diameter o~ from 0.2 to 5 ~m, advanta-geou~ly from 0.25 to 4 mm, in particular from 0.3 to 3 mm, are ad~antageously u3ed. A particularly high molding quality and good welding iB achieved if at least ome of the foam particles are smaller than the inter-connecting cavit~a~ of khe aerogel particle material.
On heating, advantageou~ly by mean~ o ~team or hot air~ the ~yren~ polymer ~of~en~, ~he foam par~icle3 expand, and the pres~ure thi~ caus~ ~ub~tantially fill9 ~he interconnecting ca~itias with the fosm, and the foam surrounds the a~rogel par~icle~ to fonm a ~trong com-.
po~ite. After cooling, the composite foam molding i~
removed from the mold and dried if necessary. ~olding~
produced in a block mold can be cut into heets u~ing a suitable cutting device.
EXAMPLES
The amounts by volume shown in the Table of finely div$ded paxticle~ o expanded poly~tyrene and hydrophoblcized silica aerogel particles ara mixed in~imately and in~roduced into a mold wh~ch does not ~eal in a gas-t$ght manner, a~ i~ cu~tomary for the production of polystyrene foam molding~. ~he mold i8 flu~hed with steam (1 bar, 100C) for 15 second~ ~fter 10 minutesl the molding ean be ramov~d and i~ ~ub~equ~n~ly dried at 40C ~or 24 hours. The results are ~hown in the Table.

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Claims (3)

1. A composite foam of low thermal conductivity, comprising a) 20 - 80% by volume of Silica aerogel particles having a mean diameter of from 0.1 to 20 mm and a density of from 0.08 to 0.40 g/cm3, b) 20 - 80% by volume of a styrene polymer foam which surrounds the particles of component a) and binds them to one another and has a density of from 0.01 to 0.15 g/cm3, and, if desired, c) conventional additive in effective amounts.

2. A composite foam as claimed in claim 1, wherein the silica aerogel particles are hydrophobic.

3. A process for the production of a compotion foam as claimed in claim 1, which comprises heating an intimate mixture of silica aerogel particle and styrene polymer foam particles to a temperature above the soft ning point of the styrene polymer in a mold which does not seal in a gas-tight manner.