Material
Com. pressive Density stress at
lb/ft3
yield
Tensile strength lb/in2
point
Ib/in*
Thermal Coeff. of conduct. linear ivity. expansion k, (50°F) per deg. F Btu/íf*in.
hoF
Expanded polystyrene
I
10
18 3-4x10-5
0.24
Table 2
TYPICAL PROPERTIES OF CELLULAR PLASTICS USED IN BUILDING
Max. tem- perature
recom-.
Water vapour diftusance 1 in. thick
board* (650 F) lb/ft*
h atm.
Behaviour in fire
Softens and collapses.
Ther- mal resisti-
vity,
mended for con- tinuous
l/k -
7 days' water absorp- tion vol.%
operation
OF
4.2
175
3.0
0.011
Flame retardant
Expanded polystyrene
1.5
18
28
4x10-5 0.23
4.4 175
2.5
0.007
grade available
Expanded polyvinyl chloride 2.5
40
3
70
3x10-5❘ 0,24
4.2 150
3.0
0.0038
!
Expanded polyvinyl chloride ¦ 4.5
130
230
3x10-5
0.24
4.2 150
3.8
0.002
Collapses and generally burns with difficulty
Foamed urea-formaldehyde
0.5
Very
Very
5x10-5 0.2.4
4.2
212
(U.F.)
low
low
Fairly 0.136
high
Resistant to ignition
Foamed phenol-formaldehyde
2
8
0,6x10-5
0.28
3.6
266
High
0.140
(P.F.)
Foamed phenol-formaldehyde 3-5
Highly resistant to ignition
34
19 0,6x10-5
0.27
3.7 266
60
0,115
(P.F.)
Foamed polyurethane
| 2-5
40
(carbon dioxide blown)
30-50 1.5-3.5%
10-5
0.27
3.7
225
4.3 0.096
Foamed polyurethane
(fluorinated hydrocarbon 1.5 blown)
Generally inflammable but relatively flame retardant forms are available
25
40-801-4X10-5
0.17
5.9
210
5-5
0.021
Expanded ebonite
40
40
3X10-5 0.20 5.0
122
1.5
0.0002
Flame retardant
*0,005 is considered a suitable value for a vapour barrier in building applications
then nailed in position inside the shuttering before the concrete is poured. Subsequently, the expanded polystyrene can be cut out. or more easily burned out with a blowlamp. The technique has also been applied to producing patterned decorative surfaces on concrete.
Expanded polystyrene has also been employed, when cast in situ. as a filler for expansion joints. though there is some doubt as to whether its resilience is sufficient for this ap plication. Also. in sealing such joints, care is necessary in the choice of sealer: hot-applied bitumen. for example. will soften and cause col- lapse of polystyrene.
EXPANDED P.V.C.
is
Although p.v.c. is a comparatively inexpensive plastics material, the pro. cess of expanding it is not simple and expanded unplasticized p.v.c. dearer than most other cellular plas- tics. Consequently its application to building is at present limited to situations where its particular pro- perties are of prime importance. It can be made with a strength and rigidity considerably greater than those of any other cellular plastic, it has an almost completely closed-cell
118
structure and a correspondingly low water vapour transmission and water absorption. and it is self-extinguish- ing without the need for fire-retardant additives. Opposed to these ad- vantages. it is sensitive to many of the solvents used in adhesives and its maximum service temperature is only 150°F.
Its low vapour transmission would appear to provide advantages where condensation might be a problem but. in fact, the most useful applica. tions seem to be in sandwich panels. where its rigidity contributes greatly to strength. In curtain-wall panels it must be used with caution. because the thermal insulating effect boards fastened directly behind cladding sheets can, in warm shine. produce surface temperatures higher than the material can tolerate. It has found more application in structural panels for boats and in cold stores, where its properties can he used to good advantage.
FOAMED
UREA-FORMALDEHYDE
sun-
Techniques have been available for some time
time for producing foamed urea-formaldehyde resins. but as the foams do not have very good me-
chanical properties there has been little interest in their use in building. However, in recent years a process has been developed for foaming these resins on the site and injecting the foam into cavity walls to improve thermal insulation. Resin which in- corporates a foaming agent is frothed up and, just before it is injected into the cavity. a hardener is added.
-
-
pre-
may
which causes the resin to set when it has been placed. As the resin is comparatively. inexpensive, consider- able improvement in thermal insula- tion is possible for little outlay, but in using the technique the original function of the cavity wall vention of rain penetration be defeated. In setting. the foam shrinks appreciably and this is likely to produce cracks and fissures in the filling. In any case, only under well- controlled conditions is it possible to fill completely all the voids between points of injection. The result is that bridges are formed at the voids be tween the inner and outer leaf of the cavity, and these increase the risk of rain penetration. While this risk might be acceptable in areas where the driving rain index, as described in Digest 23 (second series), is low or even moderate or. with a rather higher index. where the risk of pene-
THE HONG KONG & FAR EAST BUILDER-VOLUME 18, NUMBER 6
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