Page 33 - Plastics News May 2018

FEATURES
FeAtures

Flame retarded self-reinforced polypropylene composites prepared by

injection moulding

Dániel Vadas Ákos Kmetty Tamás Bárány György Maros

It is demonstrated that the combination of polymer fibre reinforcement with other fillers (such as FRs) is
advantageous and feasible even by injection moulding, but the balance between the individual attributed
properties needs to be found.

method was elaborated to prepare flame retarded self-reinforced polypropylene composites by injection moulding.
A The effect of intumescent flame retardant (FR) loading was comprehensively investigated on the morphology,
flammability, and mechanical properties of the self-reinforced polypropylene composites and compared with non-
reinforced counterparts of identical additive contents. Homogeneous distribution of both the reinforcing polypropylene
fibres and the FR additive was achieved endowing significantly improved strength and stiffness accompanied with
increased fire resistance. At 10% FR content, 37% reduction of peak of heat release rate, UL94 V-2 rating, and LOI of
24.5 vol% were achieved without compromising the improved mechanical performance of the injection moulded self-
reinforced composites, ie, a 4-fold increase of tensile strength and a 7-fold increase of tensile modulus were reached
compared with the non-reinforced counterpart. At higher loading (15% FR), the FR properties improved further (LOI
of 29.5 vol% was reached); however, due to the abrasive effect of the FR particles in the matrix, the structure of the
reinforcing fibres tends to get damaged during the thermo-mechanical processing, and thus their reinforcing efficacy
deteriorated. It is demonstrated that the combination of polymer fibre reinforcement with other fillers (such as FRs) is
advantageous and feasible even by injection moulding, but the balance between the individual attributed properties
needs to be found.
1 INTRODUCTION achieved by thermal means, is complicated by the narrow
window between the melting temperature of the matrix
Self-reinforced polymer composites (SRPCs) are and that of the reinforcing fibres.4 One possible way
composed of similar or identical polymeric materials to disturb the regularity of the macromolecular chain
for both reinforcement and matrix. This concept and thus lower the crystalline melting temperature is
provides advantages in terms of interfacial properties, copolymerization. Using a block or graft copolymer as
mechanical performance to weight ratio, recyclability, and matrix giving constituent fairly wide processing window
environmental impact;1 therefore, SRPCs gain increasing can be ensured for SRPC preparation, wide enough
interest in both academia and industries.
even for injection moulding technology.5 In the case of
Nowadays, SRPCs are mainly produced by 3 processing polypropylene, the increasing ethylene copolymer content
methods: hot compaction, consolidation of coextruded is also positively correlated to the increase of the impact
tapes, and film stacking.2 All of these methods yield in strength.6 Sterzynski et al used co-extruded composite
sheet-like (pre) products; these ways 3-dimensional parts pellets7 and bicomponent fibres8 as input materials for
with complex geometry cannot be directly produced. injection moulding of SRPCs, but to achieve homogeneous
Therefore, increasing attempts are being made to distribution of reinforcing fibres in the injection moulded
produce SRPCs by the more versatile injection moulding products proved to be rather challenging, and due to
method. Polymer fibres with highly aligned molecular undesired fibre aggregations the reinforcing efficacy
or supramolecular structure,3 generally created during remained moderate in both cases. Kmetty et al proposed
spinning or drawing, can serve as suitable reinforcements to use pre-impregnated pellets, prepared by film-stacking
in the structurally similar polymer matrix. One of the of high tenacity polymer fibres and copolymer matrix,
greatest challenges in using thermoplastic fibres to serving as adequate starting material for injection
reinforce thermoplastic matrices is in their combination moulding of SRPCs with homogeneous fibre distribution
into a well-consolidated composite, which, normally and improved mechanical properties.5, 9Recently,

33 May 2018 Plastics News