Capillary Effects in Fiber Reinforced Polymer Composite Processing: A Review

Capillarity plays a crucial role in many natural and engineered systems, ranging from nutrient delivery in plants to functional textiles for wear comfort or thermal heat pipes for heat dissipation. Unlike nano- or microfluidic systems with well-defined pore network geometries and well-understood capillary flow, fiber textiles or preforms used in composite structures exhibit highly anisotropic pore networks that span from micron scale pores between fibers to millimeter scale pores between fiber yarns that are woven or stitched into a textile preform. Owing to the nature of the composite manufacturing processes, capillary action taking place in the complex network is usually coupled with hydrodynamics as well as the (chemo) rheology of the polymer matrices; these phenomena are known to play a crucial role in producing high quality composites. Despite its importance, the role of capillary effects in composite processing largely remained overlooked. Their magnitude is indeed rather low as compared to hydrodynamic effects, and it is difficult to characterize them due to a lack of adequate monitoring techniques to capture the time and spatial scale on which the capillary effects take place. There is a renewed interest in this topic, due to a combination of increasing demand for high performance composites and recent advances in experimental techniques as well as numerical modeling methods. The present review covers the developments in the identification, measurement and exploitation of capillary effects in composite manufacturing. A special focus is placed on Liquid Composite Molding processes, where a dry stack is impregnated with a low viscosity thermoset resin mainly via in-plane flow, thus exacerbating the capillary effects within the anisotropic pore network of the reinforcements. Experimental techniques to investigate the capillary effects and their evolution from post-mortem analyses to in-situ/rapid techniques compatible with both translucent and non-translucent reinforcements are reviewed. Approaches to control and enhance the capillary effects for improving composite quality are then introduced. This is complemented by a survey of numerical techniques to incorporate capillary effects in process simulation, material characterization and by the remaining challenges in the study of capillary effects in composite manufacturing.

Low stress acrylated hyperbranched polymers

Lars Schmidt

The objective of this study was to investigate the behavior of highly functional acrylates, during isothermal ultraviolet (UV) curing. The materials included a pentafunctional acrylate and two acrylated hyperbranched polymers, one with a stiff polyester core and one with a more flexible polyether core. In particular, the influence of UV intensity and reactive blend composition on structural transitions, such as gelation and vitrification, and on the dynamics of internal stress was considered. Curing kinetics were studied with photo differential scanning calorimetry. The chemical conversion was analyzed using an autocatalytic model and a criterion for identifying vitrification directly from photocalorimetric experiments was proposed. It was observed that reactive blends containing HBPs had a higher conversion at vitrification, compared to the pure penta-functional acrylate. Strong intensity dependence of the maximum conversion rate and a weak intensity dependence of the ultimate conversion were observed. The latter was found to be controlled by the conversion at vitrification. The structural transitions and the modulus build-up during UV polymerization were determined by photorheology. A refined data processing algorithm was developed, that allows monitoring the shear modulus over 5 orders of magnitude within a short experimental time scale, with millisecond time resolution. Gelation – the liquid-solid transition – was found to be below 5 % conversion for all acrylates investigated. In contrast, the conversion at vitrification was strongly dependent on the actual monomer and increased with increasing UV intensity. The results of the photo DSC and the photorheology study were synthesized in the form of timeintensity- transformation diagrams. The dynamics of internal stress and cure shrinkage were studied using beambending and an interferometry-based method, respectively. The internal stress of the acrylated HBPs was largely reduced compared to the standard highly functional acrylate monomer. Moreover, in the case of one HBP with a polyester core and a reactive blend of the HBP with the standard highly functional acrylate, the stress reduction was obtained with a combined increase of Young's modulus, which was attributed to retarded modulus build-up and a higher final conversion. It was found that curing at a lower UV intensity led to earlier vitrification, hence earlier internal stress build-up, but limited maximum conversion thus limited final stress. Curing at a higher intensity led to later stress build-up but higher final stresses. Polymer microstructures were fabricated from the different acrylates in a photolithographic process and compared to SU-8, an epoxy frequently used for this kind of application. It was shown that the shape accuracy is linked to the processinduced internal stresses: the best result for thick and high aspect ratio microstructures – as used for example for microfluidic devices – was obtained for the acrylated HBP with a polyether core.

EPFL2006

Self-Healing Fiber-Reinforced Composites with Tailored Supramolecular Matrices

Federica Sordo

Fiber Reinforced Composites (FRCs), due to their intrinsic heterogeneity, are generally affected by complex multi-scale failure mechanisms leading to the formation of matrix cracks that are difficult to detect and repair. FRCs with the ability to heal and recover at least part of their initial properties after damage could present a solution to extend their lifetime and reliability. A possible approach is to introduce external healing agents in the matrix of the composite, through capsules or micro-channels, but this is often difficult to achieve in a high volume fraction reinforcement composite. An alternative approach is to rely on intrinsic self-healing polymers as matrices, which alleviates the need for additional phases beyond matrix and reinforcement. The present work investigated this second approach, by exploring for the first time the feasibility of processing glass fabric reinforced composites based on supramolecular hybrid network self-healing matrices and analysing their resulting properties and transfer of the self-healing ability to the composite. Hybrid networks based on epoxy chemistry, combining covalent cross-links and cooperative reversible hydrogen bonds, were selected as benchmark materials. These systems were charac- terized, in particular in terms of processing parameters and self-healing ability, and showed to be a-priori compatible with FRCs manufacturing techniques and demonstrated higher healing efficiency by increasing the H-bonds content in the polymer network, from none to 50%. On the other hand, networks characterized by large number of physical cross-links exhibited limited solvent and creep resistance, hindering their potential application. These issues were addressed by modifying the average functionality of the monomers through the combination of bifunctional and tetrafunctional epoxy resins in the compositions while keeping the use of a catalyst, leading to better controlled networks, both in terms of kinetics and final structure, and to propose a range of materials with several levels of compromise between self-healing efficiency, mechanical strength, creep resistance, and damping properties. In particular, 50% to 100% self-healing recovery of tensile properties were observed after one day, depending on the tetraepoxide content. In parallel, since the Tg of benchmark supramolecular hybrid net- works was in the range from 11-16 âŠC, an aliphatic diepoxide was introduced in the synthetic procedure to propose materials with a Tg down to -10 âŠC, to reach applications closer to those of elastomers. In view of FRCs development, the ability of partially supramolecular self-healing polymers to bond to inorganic reinforcements and to heal interfacial failure was then investigated: two dedicated butt joint-like and tack-like test methods were developed. Adhesion strength of the polymers cured on glass substrates decreased with increasing H-bonds content in the polymer networks (from 0 to 50 %) as the mechanical strength of the polymer also decreased, while the failure mechanism shifted from adhesive to cohesive due to the possibility to form hydrogen bonds with the glass substrates. Interface restoration through healing of the polymer matrices however increased from none to a strength recovery up to 70 % after 1 h healing time for the 50% H-bond polymer...

EPFL2016

Methodology for evaluating new high volume composite manufacturing technologies

Simon Jespersen

Composite materials are increasingly used in high volume automotive applications, usually as a replacement for assemblies of multiple metallic parts such as stamped and spot-welded steel sheets. One of the motivations is to reduce tooling and assembly costs, in particular at lower production volumes. Other key driving forces are increased styling freedom, reduced system cost and weight reduction. With soaring fuel prices and environmental focus on CO2 emissions, reducing vehicle mass is presently a major concern for the automotive industry. Nearly all the composites used for high production volume automotive applications today are based on short or medium length random glass fibre technologies. However, there is a great interest in extending the range of materials and processes to advanced composites (continuous aligned fibre and high fibre volume fraction materials) as they would enable superior mass specific properties compared with metals, resulting in substantial weight savings. Advanced structural composites have, however, not seen widespread use in high volume automotive applications due to a series of inhibiting factors. These typically comprise higher cost and risk when compared with standard metal forming solutions. Novel composite technologies target some of the current drawbacks, but need to mature before being widely accepted in the high volume market. It is therefore necessary to not only evaluate and develop these technologies further, but also to propose practical tools to evaluate their performance and risk level, until they reach a state where their mechanical and economic performance clearly outweighs the risks associated with their implementation. The present work aims at developing a methodology to facilitate the evaluation of new composites manufacturing technologies, while at the same time decreasing the risks associated with implementation. A second objective is to identify suitable composite material and process technologies, which will offer competitive economic and performance gains. An evaluation methodology is thus proposed and validated with a practical case study based on the analysis of two novel structural composite processes for high volume automotive components. A spare wheel well was chosen as the demonstrator since it is a structural component and is assembled as a bolt-on part, making integration within the vehicle easier. Weight assumptions, which are used as performance criteria for all examined materials, have been obtained through extensive design and FEA studies for both strength and stiffness based criteria. Cost, discounted part price, investment and other figures of merit are used as economic performance criteria and have been established through novel technical cost evaluation techniques. The presented methodology manages uncertainties and evaluates performance, and is used as a basis for establishing relations between opportunities and risks. To achieve this, tools are introduced which support the evaluation methodology, notably: Risk based costing, including probabilistic distributions for input data to measure the quality of the results, Discounted technical economic modelling, including time value of money in the technical cost assessment to point out commercially focused effects of investments, discount rates and returns, Multi-objective optimisation, providing improved control with trade-off situations. For the case study, established as well as emerging processing technologies have been evaluated to serve as benchmarks for the study of two novel composite technologies. The analysis showed that the economic performance of currently used composite technologies is severely impeded by a high material cost in the final products and in some cases by long cycle times. It is also shown that the lower investment, when compared with metal stamping, can make such technologies favourable, especially for smaller series parts, despite the base material cost being higher. New technologies showed potential for improving the economic performance further; however, high discount rates due to risk impede commercialisation. Composites generally improved the weight performance compared with mild steel, especially for strength based designs. For the spare wheel well, the processing limitations, in particular the minimum achievable part thickness, were found to limit weight reduction substantially, giving unnecessary high properties. Thermoset materials showed more consistent weight reduction between low and high temperatures, followed by PA and then PP based composites. In conclusion, the technology assessment methodology presented here is intended for implementation in an industrial environment, where it will define common objectives facilitating better communication between engineering and management. This will in turn lead to an improvement in the efficient use of resources. In addition, it has been used to identify and develop supporting tools to aid assessment of technologies and to show techno-economic effects that are valuable in an industrial context. Finally, emerging composite technologies, where the base material constituents are combined together in a direct forming process, have been evaluated. The results have shown how the supply chain can be streamlined by avoiding for example, separate weaving and needling steps, improving the overall cost performance and providing the system supplier with improved leverage.

EPFL2008