GENTRAAL LABORATORIUM
COMMUNICATION NO. 26O
CHANGES OF HUMIDITY
INSIDE PACKAGES DUE TO
ENVIRONMENTAL CONDITIONS
by
W. HAZEU
and
H. J. HUECK
Reprinted Jrom
Soc. chemical lndustry. Monograph Series
23
(19661 224
- 23t
Proc. Symposium microbiological Det6rioration in the Tropics.
London 1965.
^qil?NAIL
43
LABORATORIUM TNO, P.O. BOX 217, DELFT, THE NETHERLANDS
CHANGES OF HUMIDITY
INSIDE PACKAGES DUE TO
ENVIRONMENTAL CONDITIONS
By W. HAZEU and H. J. HUECK
(Central, Laboratory TNO, Delft, The Netherland.s)
Laboratory experiments were carried out on the influence of
exterior conditions on the relative humidity inside closed
packages.
In a first series of experiments, plastics packages containing
wool or cotton were stored at constant, high humidity and
temperature. The relative humidity inside the packages showed a
gradual increase with time; this seems to be a rather long-term
effect.
In a second series of experiments, the influence of temperature
fluctuations was studied. To prevent any loss of moisture, cotton
and wool were packed in glass containers. Only limited variations
in relative humidity, occurring within one day, were found.
In a third series o{ experiients, a tempeiature gradient of
5-15" was maintained between the top and bottom of the
contents. Under these conditions, considerable differences in
relative humidity for several parts of the package could be
observed within a few days.
A practical consequence of these findings is that, when
goods are packaged with water-impermeable materials, a rather
large safety margin must be maintained with respect to the
critical relative humidity level at which mildewing may occur.
Introduction
IN the last 20 years or so a great number of new packaging materials
of low water-vapour permeability have been developed.
These
developments have offered a solution for many packaging problems, but have created new problems in a number of instances.
In the following, some observations and experiments concerning
the behaviour of the relative humidity inside packages under
influence of exterior conditions will be discussed from the point of
view of the microbiologist interested in the conditions under which
biodeterioration of materials may occur. It is not the authors'
intention to give a thorough treatise on the packaging aspects of
the problem.
From previous studies on biodeterioration of materials, the
following practical cases of microbial damage, in which packaging
aspects may have been of importance, were known:
(1) fungal damage of documents packed in polyethylene bags
during transport by sea to the tropics;
224
w. HAZEU AND H. J. HUECK
225
(2) deterioration of an inflatable life-raft on a ship during service
in tropical regions. The raft had been packed in a bag made
of canvas;
(3) heavy mildewing during storage of raw wool packed in
polyethylene bags;
(4) spoilage of bloodmeal packed in polyethylene-lined jute bags
during transport by sea from South America to Rotterdam.
Other cases of minor importance could be added to this list.
From these observations the impression was gained that in a
number of cases the water-impermeable packaging material might
have accelerated biodeterioration.
The problem of psychrometric behaviour in closed packages has
been studied by Mueller,l who showed that the type of goods
packed had an overriding efiect on the changes of relative humidity
in the package due to changes of outside temperature. The air seems
to act only as a moisture carrier. The water contained by the air can
in general be neglected in comparison with the total amount of
water present in the goods.
A mathematical model for the calculation of the storage-life of
packed goods is given by Heiss.2 He states that impermeable
packaging materials are not always advantageous, e.g., when the
critical relative humidity value of the goods may be exceeded
through an increase in environmental temperature. On the other
hand, an impermeable packaging material is of importance for the
protection of goods against high exterior relative humidity.
Therefore, it will depend on the type of goods, and on the storage
conditions, what type of packaging material has to be chosen.
Because of our interest in biodeterioration of materials, a number
of preliminary investigations was carried out, aiming at the study
of:
(1) changes of relative humidity inside packages during storage
at constant temperature and hwmidity;
(2) changes of relative humidity inside packages due
p er atur e f,uctuati
o
lo
tem-
ns;
(3) behaviour of relative humidity inside packages under the
influence of an outside ternPeratule grad.ient.
Experimental
(1) Changes of relatiae humidity ,inside packages during storage at
constant temperature and humidity
Storage experiments at 30o and 90% R.H. were carried out with
cotton and woollen fabrics, packed in 0.1 mm thick polyethylene
w. HAZEU AND H. J. HUECK
or PVC film. The dimensions of the packages were approximately
15 x 20 cm, the polyethylene being heat-sealed, the PVC high226
frequency sealed.
The packages contained 120 g of cotton or 100 g of wool, and
inside each was attached a moisture-sensitive element (El-tronics).
These elements consisted of a specially prepared sheet of polystyrene, with an electrode printed on the surface. Two insulated
wires, connected with the element and sealed in the seams, protruded from the package.
The electrical resistance of the elements (which is a function of
temperature and humidity) was measured and the corresponding
value of the relative humidity was read from a calibration curve.
This value was corrected for temperature, if necessary. No control
experiments as to the impermeability of the sealing were performed.
(2) Changes
oJ relatiue
humidity inside packages due to temperature
fl,uctuations
The experiments were carried out in 2-l measuring glasses, filled
with cotton wool or wool of different moisture contents (regains).
The glasses were closed with a paraffin-coated cork to prevent loss
of moisture. Each glass was equipped with a thermometer and two
moisture-sensitive elements. The temperature range from 20 to 40'
was studied.
The glasses were incubated at constant temperature until the
equilibrium relative humidity was reached. This took usually less
than 24h.
(3) The behaaiour of relatiae humid,ity inside packages under
the
infl.wence of an owlside temt'erature grad.ient
The experiments were carried out in measuring glasses as described above. Each glass contained two moisture-sensitive elements and two thermometers, one near the bottom and one near
the top. The glasses were filled with 115g of cotton or 1009 of
wool of different regains, and closed. They were placed with their
lower half in a constant-temperature water-bath, the upper half
protruding above the water surface, in air of a constant temperature of 23". The temperature of the water-bath was adjusted so as
to maintain a constant temperature-difference inside the package
between the lower and the upper parts of the contents of
approximately 5, l0 or 15'. The relative humidity in both
parts of the glasses was determined daily until equilibrium was
reached.
w. HAZETI AND H. J. HtTECK
227
Results
Storage experiments at constant temperatwre and relatite hwmid.ity.
The results of the storage experiments are given in Table
I.
The
variation in the relative humidity values is*3o/o R.H. or less. The
impression was gained that the elements showed some change with
time, so that periodic calibration appeared to be necessary. Hence
the results obtained should be considered on only a comparative
basis.
R e t at i
a
e
hu
midit
Packaging material
y (%) *'
Table I
;32
ll::
gt Xi"#,
;,
0'l mm polyethylene film
Packed goods
d
i ff e r e n t
sto
r ag
e
p
e
ri
o
ds
0.1 mm polyvinyl chloride
film
1[roollen
fabric
AIter
0 days
20 days
43 days
99 days
l2l days
Infl,uence
of
packages
4t
5I
42
47
56
65
57
59
66
47
54
68
74
50
60
74
79
86
temperature fl.uctuations
aq
8l
on relatiae hucnidity inside
The results of the experiments with cotton and wool are given
in Figs. 1 (a) and (b).
2
s
'-{-ar<<.26
>5
F
o
l
u
F
U
G
r<'*'e""
t,[
TEMPERATURE,.C
Fte .
l.
Relation belween relatiue humidity and, teruperature al
constant rega'in for (a) cotton, (b\ uool
Numerals on curves are o/o moisture
w. HAZEU AND H. J. HUECK
228
Behauiowr of relatiue loumid.ity inside packages under the inf.uencc of
an outside temperatwre gradient
The results of the experiments with cotton and wool are summarised in Figs. 2 (a) and (b), where the relative humidity at the
beginning of the experiment is shown on the abscissa and the
equilibrium relative humidity, at a constant temperature difference
between top and bottom of the package, on the ordinate.
Discussion
Storage experiments at constant temperatwre and humidity
The results of the storage experiments, given in Table I, show
that the relative humidity inside the packages increases gradually
with time. This change will depend on the storage conditions, on
the type of goods packed and on the water-vapour permeability of
the packaging material.2, 3 It is concluded that the relative
humidity inside packages during long-term storage under humid
conditions can increase to a level at which mildewing may occur.
When the goods are packed at a sufficiently low water content, this
will take a rather long time, depending on the type of packaging
material used.
Influence of temperatwre fluctuation on relatiae humid.ity
The influence of temperature on the equilibrium relative
humidity of wool and cotton at constant regain has been discussed
by many authors, among others by Hearle & Petersa and in the
book entitled 'Wool Research'.5 From these publications it appears
that the relative humidity in equilibrium with wool or cotton at
s
'*'*'t"W
o
=
l
I
u
k
u
G
z
T,^,
T
,---{
'%
b'-
High t.mp.
r.9lon
L
Erc 2. Relatiue humid,ity .inside pachages at a
consl@Mt tetnperature
difference betaeen top and, bottom for (a\ cotton, (b) waol
Numerals on curvgs are temperatqla difipteaces
w.
HAZEU AND H.
j. HUECr{
229
constant regain increases with temperature. This is caused by the
liberation of physically bound water, which phenomenon has been
considered thermodynamically by the authors mentioned above.
As can be seen from Fig. 1 this effect is only moderate. It is
established
within
24 h.
Therefore, fungal damage can be expected only when the initial
relative humidity is only slightly below the critical level. The
values observed show fairly good agreement with those reported in
the literature, allowing for experimental error.
The inf.uence of a temperature grad,ient
In these experiments, the temperature in a part of the package
was raised, causing an increase in the water vapour pressure. fn
this situation, water vapour will migrate to the part at lower
temperature to cause a local increase in the moisture content of
the packed goods. This will go on until equilibrium is reached,
when the partial water vapour pressure is equalised over the whole
package, a process which only took a few days. The result is an
increase in relative humidity at the lower temperature region, and
a decrease at the higher temperature.
As can be seen from the experimental results (Fig. Z), the local
increase in relative humidity is considerable at a rather low temperature difference. The results obtained can be compared with
some expected values given in Fig. 3. These graphs have been
s
tr
o
f,
I
U
t
z
L
30609003060
tNt-ilAL RELATtvE
HUMiDtTy, yo
I're . 3. Expected, relatiue humid.iti,es i.nside pachages
a.l a constant
temperature difference betueen top and bottom Jor
(a) cotton, (b) u.tool
Numerals on curves are temperature differences
W. IIAZDU AND H. J. HUECK
constructed from a theoretical model in which the contents of the
package are divided into two equal parts, each having a constant,
but different, temperature. The overall mean regain of the contents
has been considered to be constant, the water content of the air
being neglected.
The equilibrium water-vapour pressure at a constant temperature difference has been estimated from graphs, representing the
relation between water-vapour pressure and regain at constant
temperature, derived from data given in the literature.a-G These
graphs can be constructed for a number of initial temperatures.
Clearly, the changes observed show reasonably good correlation
with those expected, albeit somewlrat smaller. The differences can
be explained by the actual situation in the package, in which the
temperature gradient was not as ideal .s wus aisumed for the
model.
The efiects observed are expected to be more pronounced when
a relatively smaller part of the contents of the package is at the
lower temperature, whereas a less serious situation exists when the
reverse is true.
As shown by the Figures, the relative humidity may reach the
critical region at which fungal growth is possible. A value of 70o/o
has bcen reported to be the lowest value at which fungal growth
can be expected, although this will seldom occur in practice.T-s
It should be realised, however, that the probability of fungal
growth increases with the relative humidity, dependent on the
type of material. At a temperature difference of 15", e.g., an 80o/o
R.H. value is reached for both cotton and wool packed at around
60% R.H. Situations under which temperature fluctuations and
differences of the order studied above occur, may actually exist
in practice.
In this respect, reference is made to the studies by Mielke,lo who
reported considerable fluctuations in temperature and relative
humidity in a cargo during a voyage to and from tropical areas;
the maximum temperature fluctuations could be as great as 30o
or more, while the relative humidity often exceeded 90o/o.
Therefore, it must be concluded that a rather large safety margin
has to be maintained with respect to the critical relative humidity
of cotton and wool at which mildewing may occur. This is especially
true for situations that may exist during storage of these materials
in, or sea-transport to, tropical areas.
Conclusions
(1) The relative humidity inside plastics packages changes
231
J. HUECK
gradually during storage, dependent on the conditions, type of
w.
HAZEU AND H.
packaging material, type of goods packed, etc. These changes may
be long-term effects, however.
(2) Rises in temperature cause a moderate increase
in relative
humidity through the liberation of bound water from the packed
goods, within a relatively short time.
(3) A temperature gradient within the contents of the package,
on the contrary, may cause a considerable local rise in relative
humidity within a few days.
(4) A rather large safety margin should be maintained with
respect to the moisture content of wool or cotton, at which
mildewing may occur during sea-transport or storage.
References
1 Nlueller, M. F., Mod,. Pachag., 1949, 22, (11), 163
2 Heiss, R., Verpach.-Rdsch., Franhf .,1957, 8, (3), 17
3 Hannan, R. S.,
J. appl. Bact., 1962,25r248
a Hearle,
J. W. S., & Peters, R. H., 'Moisture in Textiles', 1960, p. l4 (London:
Butterworths)
5 'WooI Research', I955, Vol. 2, p.20 (Leeds: Wool Industries Research Ass.)
6
Jeffries, R., J. Text. Inst., 196O, 51, T 339
7 lllman, W. I., & -Weatherburn, M. W., Am. Dyestulf Reptr, 7947, 36,343
8 Snow, D., Ann. appl. Biol.,1949, 36, I
e Block, S. 5., APpl. Microbi.ol., 1953, 1,287
r0Mielke, H., Verpach.-Ildsch., Franhf., 1962, 13, (12), 93; 1963, 14, (10), 75;
Neue Verpach., 1962,75,1239; 1963, 16, 1107