Today, in the time of climatic change, natural fibre-reinforced
plastics are an interesting alternative to composites based on glass and petrochemical fibres. Due to their good
lightweight construction potential and good crash characteristics,
they are used more and more as reinforcement for
construction materials. Nevertheless, application is often
limited to non-visible components in the automobile industry
[1] or to products for which there is little demand like salvers
or briefcases. In order to conquer further areas of application
and to satisfy the increasing demand for natural composites,
new high-quality materials need to be designed, and further
material combinations need to be tested.
The strength of the bond of fibre and matrix is substantial
for the mechanical properties of a composite. Fibre/
matrix adhesion is a complex process with many factors
interacting with each other. It can be affected by the choice
of materials, manufacturing methods, processing parameters,
surface treatments of the fibres or by additives like
adhesive agents. Thus, several studies dealing with the
optimisation of fibre/matrix adhesion in natural fibre-reinforced
composites have already been conducted.
Bhat et al. examined the adhesion between fibres and
different thermoplastic matrices. For this purpose, they
produced cotton composites with matrices in the form of
foils or fibres. Then they examined the composites’
mechanical properties; the best fibre/matrix adhesion and the
highest composite strength could be achieved by using a
matrix in the form of fibres. In addition, the processing
parameters during production have a decisive influence on
the composite properties [2]. For example, Mu¨ ller et al.
examined the influence of the processing temperature on the
strength of natural fibre-reinforced thermoplastics. The
result is a temperature curve which indicates that a too low
press temperature (too high viscosity of the matrix) as well as
a too high press temperature (start of fibre degradation) leads
to a decreased composite strength [3]. Likewise, pre-treating
the fibres with different chemicals has a strong influence on the composite properties. Mehta et al. analysed the consequences
of different surface treatments like alkalis, silanes
and acrylonitriles. The test was carried out with hemp needle
felt/unsaturated polyester resin (UP) composites. It is quite
evident that mechanical and thermal composite properties
could be increased tremendously by these surface treatments
[4]. Joffe et al. examined fibre/matrix adhesion with the
fragmentation test. Their results also show that fibre/matrix
bonding can be increased by fibre treatment [5]. Endres et al.
investigated whether maleic anhydride-grafted (MAH-g)
coupling agents improve the mechanical properties of
injection-moulded natural fibre-reinforced thermoplastics.
They found out that the mechanical properties of these
composites could be improved by up to 40% with an optimised
coupling agent content as well as with an optimal
MAH-content proportional to the fibre surface [6] Beside the fragmentation test, microscopic examination
can be used for evaluating fibre/matrix adhesion. Michaeli
et al. tested different microscopic examination methods—
i.e. scanning electron microscopy (SEM), transmission
electron microscopy, energy-dispersive X-ray analysis and
atomic force microscopy—to evaluate the bonding surface
in flax fibre-reinforced thermosets [7].
Today, in the time of climatic change, natural fibre-reinforcedplastics are an interesting alternative to composites based on glass and petrochemical fibres. Due to their goodlightweight construction potential and good crash characteristics,they are used more and more as reinforcement forconstruction materials. Nevertheless, application is oftenlimited to non-visible components in the automobile industry[1] or to products for which there is little demand like salversor briefcases. In order to conquer further areas of applicationand to satisfy the increasing demand for natural composites,new high-quality materials need to be designed, and furthermaterial combinations need to be tested.The strength of the bond of fibre and matrix is substantialfor the mechanical properties of a composite. Fibre/matrix adhesion is a complex process with many factorsinteracting with each other. It can be affected by the choiceof materials, manufacturing methods, processing parameters,surface treatments of the fibres or by additives likeadhesive agents. Thus, several studies dealing with theoptimisation of fibre/matrix adhesion in natural fibre-reinforcedcomposites have already been conducted.Bhat et al. examined the adhesion between fibres anddifferent thermoplastic matrices. For this purpose, theyproduced cotton composites with matrices in the form offoils or fibres. Then they examined the composites’mechanical properties; the best fibre/matrix adhesion and thehighest composite strength could be achieved by using amatrix in the form of fibres. In addition, the processingparameters during production have a decisive influence onthe composite properties [2]. For example, Mu¨ ller et al.examined the influence of the processing temperature on thestrength of natural fibre-reinforced thermoplastics. Theresult is a temperature curve which indicates that a too lowpress temperature (too high viscosity of the matrix) as well asa too high press temperature (start of fibre degradation) leadsto a decreased composite strength [3]. Likewise, pre-treatingthe fibres with different chemicals has a strong influence on the composite properties. Mehta et al. analysed the consequencesof different surface treatments like alkalis, silanesand acrylonitriles. The test was carried out with hemp needlefelt/unsaturated polyester resin (UP) composites. It is quiteevident that mechanical and thermal composite propertiescould be increased tremendously by these surface treatments[4]. Joffe et al. examined fibre/matrix adhesion with thefragmentation test. Their results also show that fibre/matrixbonding can be increased by fibre treatment [5]. Endres et al.investigated whether maleic anhydride-grafted (MAH-g)coupling agents improve the mechanical properties ofinjection-moulded natural fibre-reinforced thermoplastics.They found out that the mechanical properties of thesecomposites could be improved by up to 40% with an optimisedcoupling agent content as well as with an optimalMAH-content proportional to the fibre surface [6] Beside the fragmentation test, microscopic examinationcan be used for evaluating fibre/matrix adhesion. Michaeliet al. tested different microscopic examination methods—i.e. scanning electron microscopy (SEM), transmissionelectron microscopy, energy-dispersive X-ray analysis andatomic force microscopy—to evaluate the bonding surfacein flax fibre-reinforced thermosets [7].
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