Interlaminar fracture toughness improvement in composites with hyperbranched polymer modified resin
Introduction
Light plastics and composites are progressively replacing most of the non- and semi-structural metallic parts of vehicles. Structural parts, such as the floor-pan and body forming the Body-in-White (BIW) have however not been replaced yet except for few small series applications. The automotive industry has now taken the initiative to co-operate on a European level in an attempt to enable the use of carbon fibre composite materials in fully load bearing, automotive structures such as the BIW. The proposed route is to use RTM of epoxy into carbon fibre non-crimp fabrics.
The use of thermosetting materials is often limited by their toughness properties. At the temperature of use (room temperature up to 100 °C), they are in the glassy state, which confers upon them satisfactory stiffness (E ∼ 2 GPa) but in turn, makes them prone to brittle failure. This affects the durability of components and places strong constraints on the design parameters. There nevertheless exist several solutions to this problem. The introduction of spherical rubber particles within the glassy matrix has allowed significant improvement in fracture behaviour albeit to the detriment of stiffness properties. These are either introduced as preformed rubber particles, or in solution with the resin, which phase separates during cure. Usual volume fraction stands within 5–20% depending on the application [1], [2]. However, these are generally not suitable to liquid composite moulding because of the filtering effect by the fibres or the high viscosity of the infiltrating resin, depending on the initial state of the rubber.
Hyper-branched polymers (HBPs) do not suffer from this and have been shown to exhibit good characteristics when used as modifiers in polymers, as they can be miscible and phase-separate during cure, while the blend retains a low viscosity suitable for liquid moulding [3], [4]. The molecules are composed of three distinct parts: a multi-functional core, several layers of monomers and a multi-functional shell. The core dictates the mechanical properties of the HBP whereas the chemical structure of the hull grafted on the surface controls the phase separation process and can be tailored for each type of resin.
It has been shown earlier [3], [5] that HBP-modified epoxy resins display enhanced properties compared to non-modified resins. Boogh et al. [3] indicated that a 6-fold increase in GIC, corresponding to a 2.5-fold increase in KIC, could be obtained for epoxy resin using 5% by weight of a HBP modifier. Mezzenga et al. [6] pointed out the pronounced effect of the sizing on the interlaminar fracture toughness of glass fibre composite with HBP modified resins. A specific amino-silane sizing allowed an increase of 75% in GIC compared to a standard epoxy sizing.
This article presents a study in three stages to improve the interlaminar toughness of a carbon–epoxy composite processed by RTM using a phase-separated epoxy-functionalized HBP. In the first stage the correct amount of resin modifier is assessed on the neat resin, by counterbalancing the loss in stiffness and the gain in toughness. The second stage of the study intends to evaluate the influence of the HBP on the toughness of composites processed by RTM. Double Cantilever Beam (DCB) specimens were manufactured and tested. In the third stage of this work, fibre–matrix adhesion is measured and a solution to the observed loss in fibre–matrix adhesion is proposed to restore adequate composite toughness properties.
Section snippets
Materials
The resin used in the present work is a two-component thermoset epoxy, Epikote 828LV with the hardener Epikure DX 6514 from Shell. They are mixed with a ratio of 100:17 (parts by weight of base and hardener, respectively). The modifier Boltorn E1 is supplied by Perstorp AB and is a three-generation dendritic hyperbranched polyester with a tetra-functional core, and three successive layers of shell. The outer shell is functionalised with aliphatic epoxy groups with an epoxy equivalent weight
Toughness
The values of KIC and GIC, respectively, the critical stress intensity factor also known as the critical plane-strain fracture toughness and the critical strain energy release rate were calculated from the experimental curves.
Using the CT geometry, the stress intensity factor KQ and the failure energy GQ are:where PQ is the maximum load on the force–displacement curve, f(x) is a factor depending on a and W, U is the area under the force–displacement curve and ϕ is a
Results
The optimised amount of HBP was determined by studying the effect of HBP content on the neat resin only. From the work of Mezzenga et al. [6], [23], [24], [25], it has been shown that a good compromise between toughness increase and the decrease in stiffness ranges from 5% to 20%. HBP contents studied herein were thus: 0%; 7.5%; 10%; 12.5% and 20%. Finally, the correct level of modification is found by comparing the toughness increase, the loss in stiffness, and the viscosity change of the
Results
The impregnation quality of the laminates was excellent. No observable differences were observed as a consequence of the presence of the modifiers. Fig. 13 presents examples of the evolution of GIC as a function of the crack opening for the composites processed with the neat resin (a) and the HBP modified resin (b). Crack propagation within the composites with mid-plane contained between two 0° plies was relatively stable. The scatter is very small for the neat resin composites, whereas it is
Conclusions
HBPs were used to increase toughness in neat epoxy resin. An optimised concentration of 7.5% was chosen, to yield a 66% increase in terms of critical plane-strain fracture toughness. For the same level of modification, the strain energy release rate was multiplied by a factor of 3. The stiffness was retained since the flexural modulus decreased by only 10%. The curing kinetics were also slightly modified but remained acceptable for RTM processing. HBP modifiers did not however modify
Acknowledgements
The authors acknowledge Saertex GmbH for the manufacture of NCF fabrics which were supplied for this work. Perstorp Speciality Chemicals is also thanked for supplying the Boltorn-E1 HBP molecules. Financial support from the OFES (Swiss Federal Administration for Science and Education) in the framework of the European Growth project TECABS is gratefully acknowledged. Dr. Bouchet of LTC and Dr. Mezzenga are also acknowledged for fruitful discussions, as well as Bertrand Pichon for performing the
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2022, Progress in Materials ScienceCitation Excerpt :These unique properties provide improved toughness without reducing processability. In addition, a large amount of free space in HBPs is conducive to enhancing the toughness of the EPs [181–188]. Commercially available hyperbranched polyesters such as Boltron™ have been the overwhelming focus of study, which shows a good toughening effect to the epoxy matrix [189,190].