The influence of the post-curing treatment on the mechanical properties of the glass fibers mat polymeric composite materials.
Teodorescu, Horatiu ; Motoc Luca, Dana
1. INTRODUCTION
It is well acknowledged and proven by a wide spectrum of scientific
research that the performance of composite structures depends on the
constitutive elements characteristics (e.g. material types, quantity,
architecture within the structure, etc.), manufacturing technology,
working conditions, etc. Temperature influence on the mechanical
characteristics it is one of the subjected that was approached quite
extensively in the literature for different types of material
combinations and in all cases material degradation were reported
(Dimitrienko, 1999).
With respect to the subject of the paper, flexural modulus is
usually reported into the literature, for various reasons that will not
mentioned in here and temperature influence upon it comes usually as
being an external factor with an increasing trend (Cerny & Glogar,
2004), (John & Venkata Naidu, 2004).
The influences of the constitutive elements on the elastic
properties of the composite materials are another trendy subject in the
domain of composite materials characterization, usually reported from
fibre length, distribution, and volume fraction and architecture point
of view (Xu, Eichmiller & Barndt, 2001).
One of the author of the herein paper reported the influence of the
thermal treatment on the electrical characteristics of particle
reinforced polymeric composite materials, in the sense of improvements
on these characteristics. This idea was extended to other types of
composite materials, such the ones approached in this paper (Oltean
& Motoc Luca, 2008).
The present work focuses on the mechanical properties of the random
E-glass fibres composites, reinforced with different volume fraction of
mineral fillers (used to lower the costs of the materials, improve the
UV resistance and electromagnetic characteristics). More specifically,
the paper focuses on the experimental values retrieved from flexural
testing as a part of a more extensive work focused on this class of
composite materials. The samples tested were subjected to thermal
cycles, simple and consecutively, with the aim of sizing the influence
of the post-curing treatment on the mechanical characteristics of the
composite structures implied into the studies. Particular to this paper,
different from other and in the same time new is the fact that it shows
that a single relatively long post-curing thermal treatment it is enough
to lead to mechanical characteristics improvements.
The experimental results retrieved provide useful information for
design purpose regarding to the improvements that can be done especially
when the structure is being employed in aggressive media environments.
2. EXPERIMENTAL ANALYSIS
2.1 Materials
In this study were used random, long E-glass fibre mats in a
combination with a polymeric matrix from DSM Composite Resins
(Switzerland), the latter being choose due to its availability and very
good fibre wetting and impregnation properties. The
fibres--MultiStrat[TM] Mat ES 33-0-25 supplied by Johns Manville, USA,
were made up from multidirectional continuous E glass fibres, leading to
a 60% volume fraction in the composites' panels. The fillers
considered were CaC[O.sub.3] particles embedded into different volume
fraction into the composite's matrix material (5 %, 10 %
respectively).
2.2 Testing methods
The samples' mechanical properties were evaluated at room
temperature, after a single thermal cycle and 3 consecutive thermal
cycles, respectively. One thermal cycle applied, by aid of a controlled
temperature oven, had the following variation: 1 hour of heating up (5
min. until the temperature was reached) and maintaining at 120[degrees]C
followed by 1 hour of cooling down at room temperature. The samples were
shaped using a standardized form (SR EN ISO 178) and subjected to
flexural loading conditions (3 points), all of them with a crosshead speed of 1 mm/min using a LR 5K Plus device from Lloyd Instruments Ltd.
The results obtained applying a statistical analysis was considered as
corresponding to the mean values.
3. RESULTS AND DISCUSSION
The experimental results show the influence of the post-curing
thermal treatments on the mechanical characteristics of the composite
samples investigated. In figure 1 was plotted the Young modulus of
bending retrieved for samples without thermal treatment and samples
subjected to a single thermal cycle, respectively.
As can be seen, in case of the structure without mineral fillers or
with a 10% of fillers content an improvement of the flexural modulus
with 34% and 46 %, respectively, was reached. The 5% content of mineral
fillers seems to lead to an opposite effect in the modulus variation.
This behaviour was found for the values retrieved from tension and
compression testing and may help us to conclude that this percentage
represent a critical amount that has to be avoided during the composites
manufacturing.
In case of applying consecutively thermal cycles (3 in this case)
the experimental values do not reveal huge modifications with respect to
the values retrieved after one thermal cycle applied as part of a
post-curing treatment. This behaviour can be regarded to the composite
material structure, namely to the fact the previous thermal loading was
long enough to ensure the volatile compounds evaporation and
polymerization process ending.
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
The same findings can be applied to the variations of the
samples' flexural rigidity, were again there are no huge changes
due to an extra thermal treatment. The flexural stress (used in
structure designs) experience the behaviour as the flexural modulus
along with the thermal treatments applied upon the composite samples.
A weigh reduction (see table 1) and dimensional shrinkage was
recorded in case of the samples subjected to thermal treatments. No
further thermal treatments were applied in order to avoid samples
degradation or maturation during the experimental trials.
4. CONCLUSIONS
The variations of the flexural modulus and flexural rigidity of
glass fiber mat reinforced polymeric composites reinforced by various
volume fractions of mineral filler, without and after subjected to a
single thermal cycle at a temperature of 120[degrees]C, or 3
consecutively thermal cycles with the same parameters, were
investigated. The thermal influence was considered as a natural
consequence of the manufacturing process, and can be viewed as
post-curing treatment. The improvements of the flexural modulus sized
after the first thermal cycle were sufficiently to help authors to
conclude that a single relatively long thermal cycle will be enough to
improve the mechanical characteristics of any type of composite
structures, especially in industry environments were the manufacturing
and materials costs are of high concerns along with improvements on
their mechanical characteristics.
Acknowledgement
The research was supported from grant ID_135, 108/1/10/2007,
CNCSIS, Romania.
5. REFERENCES
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Tab. 1. Relatively weight loss after the thermal cycles
Composite type 1 thermal 3 thermal
glass fibre & cycle cycles
polymeric matrix
0 % mineral filler 6.0 0.01
5 % mineral filler 2.4 0.12
10 % mineral filler 4.4 0.01
