Pieter Samyn

The tribological behaviour of engineering plastics during sliding friction investigated with small-scale specimens

Abstract For economical, ecological and even technical reasons for some years there has been a tendency to introduce self-lubricating materials for bearing applications. In this way external lubricants such as oil or grease can be excluded, the design can be simplified and maintenance cost can be reduced. Among the self-lubricating materials the so-called engineering plastics have increasing importance. Unfortunately, data on their friction and wear characteristics are very disparate and often there is a lack of general understanding of the physical phenomena involved. In the present paper some basic types of engineering plastics (PA, oil-filled PA PTFE-filled PETP, POM-H) are experimentally investigated by means of small-scale reciprocating tribotesting. The tribological behaviour is explained in correlation with the chemical and mechanical properties of the materials. The basic failure processes are described for mild wear conditions as well as for overload conditions.

Wetting and hydrophobic modification of cellulose surfaces for paper applications

The use of papers in packaging and development of novel technological applications for paper substrates largely depends on the control of the hydrophilic properties of the cellulose fibres and improvement of the water-repellent properties. This review provides an actual summary of available literature on theoretical concepts and practical methods to improve the hydrophobicity of cellulose fibres and paper webs. In the first part, the interaction of water with cellulose fibres and paper webs is described at different levels ranging from the molecular scale over the micro- to macroscale fibre properties towards the interactions with porous substrates. The concepts for hydrophobicity and superhydrophobicity applied to cellulose fibres are reviewed, considering the surface chemistry and topographical features. In the second part, current techniques for hydrophobization based on sizing or direct fibre surface functionalization are described. Besides traditional sizing procedures, novelties in nanoparticle applications as hydrophobic sizing agent are reviewed. Novel trends in physical, chemical and nanotechnological fibre surface modifications or surface coatings are reviewed to turn the wetting properties into the superhydrophobic regime. The main future concern in controlling cellulose wettability lies in the development of sustainable modification techniques based on renewable resources such as biopolymers and green chemistry.

Polymerizable Biomimetic Vesicles with Controlled Local Presentation of Adhesive Functional DOPA Groups

Inspired by strong adhesive properties of mussel footprint proteins, which are largely governed by the presence of dihydroxy-phenylalanine (DOPA) amino acid moieties, we present a novel approach for presenting DOPA groups in a very defined way in order to modulate the adhesion between artificial interfaces. To this end, linear peptide amphiphiles are synthesized with attached DOPA functional groups and a polymerizable diacetylenic tail. The obtained amphiphiles can be coassembled with matrix amphiphiles into vesicles, which can be subsequently stabilized through UV-light-induced solid-state polymerization. Depending on the molar ratio of matrix and adhesive amphiphiles, the vesicles self-assemble into spherical, fibrilar, or planar nanostructures. The adhesive properties of the surface-adsorbed vesicles are evaluated by drop casting them onto a planar solid substrate and performing macroscopic shear tests in contact with a similar substrate. The shear forces are investigated as a function of substrate chemistry, vesicle polymerization conditions, vesicle concentration, and number of adhesive DOPA groups in the interface. Substrate adhesion is enhanced by surface-confined vesicles and greatly depends on the presentation of DOPA groups in the adhesive interface, either as a mono- or multilayer conformation. Because the adhesive structures can be transferred onto substrates from low-viscosity aqueous solution, they may serve as interesting nanoscale gluing pads in future applications, where the high viscosity of polymer-based glues renders the controlled formation of nanoscale adhesion pads difficult.

Quality and Statistical Classification of Brazilian Vegetable Oils Using Mid-Infrared and Raman Spectroscopy

Palm oil, soy oil, sunflower oil, corn oil, castor oil, and rapeseed oil were analyzed with Fourier transform infrared (FT-IR) and FT-Raman spectroscopy. The quality of different oils was evaluated and statistically classified by principal component analysis (PCA) and a partial least squares (PLS) regression model. First, a calibration set of spectra was selected from one sampling batch. The qualitative variations in spectra are discussed with a prediction of oil composition (saturated, mono- and polyunsaturated fatty acids) from mid-infrared analysis and iodine value from FT-Raman analysis, based on ratioing the intensity of bands at given wavenumbers. A more robust and convincing oil classification is obtained from two-parameter statistical models. The statistical analysis of FT-Raman spectra favorably distinguishes according to the iodine value, while the mid-infrared spectra are most sensitive to hydroxyl moieties. Second, the models are validated with a set of spectra from another sampling batch, including the same oil types as-received and after different aging times together with a hydrogenated castor oil and high-oleic sunflower oil. There is very good agreement between the model predictions and the Raman measurements, but the statistical significance is lower for mid-infrared spectra. In the future, this calibration model will be used to check vegetable oil qualities before using them in polymerization processes.

How Thermal Curing of an Organic Paper Coating Changes Topography, Chemistry, and Wettability

Celluloses are preferred renewable substrates, but hydrophilicity and porosity disfavor their water resistance. We present here an ecofriendly application of imidized nanoparticles and a method to flexibly tune the surface wettability of papers. The soft nanostructured coating is sensitive to thermal curing, which affects both the surface chemistry and morphology. The thermal stability of the coating is first investigated with conventional and modulated differential scanning calorimetry, revealing influences of the imide content and an endotherm reaction below the glass transition temperature at 120-150 °C. The latter is studied in detail for an appropriate selection of the copolymer precursors. According to diffuse reflection infrared spectroscopy, Raman spectroscopy, and UV/vis spectroscopy, the endotherm corresponds to an imidization reaction. The morphology of the coatings is followed at various scale levels by contactless roughness measurements and atomic force microscopy. Finally, the experimental values are fitted to the parameters of the Wenzel wetting model, and so-called calibration curves for the relation between contact angles, surface roughness, and surface chemistry are presented. They allow the prediction of the water contact angle of coated papers from the hydrophilic to the hydrophobic range, with a maximum in hydrophobicity after increasing the imide content at 120-150 °C curing.

Effect of homogenization (microfluidization) process parameters in mechanical production of micro- and nanofibrillated cellulose on its rheological and morphological properties

The rheological properties of microfibrillated cellulose (MFC)/nanofibrillated cellulose (NFC) suspensions have an important role during processing and mixing. In this work, the process parameters for MFC/NFC production within a microfluidizer (i.e., the size of interaction chamber and number of passes) were varied to investigate the influences on morphology, zeta potential, chemical properties and rheological features including viscosity, creep, strain recovery and yield stress behavior. The stability and appropriate viscosity of the fiber suspensions can be controlled by optimizing the processing conditions, resulting in a reduction in fiber diameter and most negative zeta potential value. The viscosity increased with higher amount of fibrillation by using a smaller chamber or higher number of passes, but intermediate plateau values are characteristic for temporary aggregation and breaking-up of the fiber network. The creep response and yield stress have been described by parameters of the Burger model and Herschel–Bulkley model, respectively, showing a more prominent effect on yield stress of chamber size than number of passes. The network formation leads to lower creep compliance and step-like strain recovery. The transition from gel-like to liquid-like behavior as characterized by the dynamic yield point at a specific strain, is almost independent of the processing conditions. Most important, the total number of passes applied in production can be directly related to the rotational Péclet number, which combines rheological and morphological data.

Postmortem raman spectroscopy explaining friction and wear behavior of sintered polyimide at high temperature

With their thermal stability and high strength, polyimides are an aromatic type of polymers that are used in sliding equipment functioning under high loads and elevated temperatures. However, their tribological behaviors under high temperature and atmospheric conditions are not fully understood. It has been reported that a transition from high to lower friction occurs “somewhere” between 100 and 200 °C; however, correlation with changes in the polyimide molecular structure remains difficult to illustrate, and it is not certain whether this transition is correlated to lower wear. In the present work, sliding experiments under controlled bulk temperatures between 100 and 260 °C are performed. A transition in both friction and wear at 180 °C is observed that is further examined with microscopic analysis of the transfer film on the steel counterface and Raman spectroscopy of the worn polymer surfaces. Close examination of the spectra reveals transitions in the relative intensities of certain absorption bands, which suggests different orientation effects of the molecular conformation at the polymer sliding surface at 180 °C.

Review: nanoparticles and nanostructured materials in papermaking

The introduction of nanoparticles (NPs) and nanostructured materials (NSMs) in papermaking originally emerged from the perspective of improving processing operations and reducing material consumption. However, a very broad range of nanomaterials (NMs) can be incorporated into the paper structure and allows creating paper products with novel properties. This review is of interdisciplinary nature, addressing the emerging area of nanotechnology in papermaking focusing on resources, chemical synthesis and processing, colloidal properties, and deposition methods. An overview of different NMs used in papermaking together with their intrinsic properties and a link to possible applications is presented from a chemical point of view. After a brief introduction on NMs classification and papermaking, their role as additives or pigments in the paper structure is described. The different compositions and morphologies of NMs and NSMs are included, based on wood components, inorganic, organic, carbon-based, and composite NPs. In a first approach, nanopaper substrates are made from fibrillary NPs, including cellulose-based or carbon-based NMs. In a second approach, the NPs can be added to a regular wood pulp as nanofillers or used in coating compositions as nanopigments. The most important processing steps for NMs in papermaking are illustrated including the internal filling of fiber lumen, LbL deposition or fiber wall modification, with important advances in the field on the in situ deposition of NPs on the paper fibers. Usually, the manufacture of products with advanced functionality is associated with complex processes and hazardous materials. A key to success is in understanding how the NMs, cellulose matrix, functional additives, and processes all interact to provide the intended paper functionality while reducing materials waste and keeping the processes simple and energy efficient.

Hybrid palm-oil/styrene-maleimide nanoparticles synthesized in aqueous dispersion under different conditions.

Abstract Poly(styrene-co-maleic anhydride) was imidized with ammonium hydroxide and palm oil, resulting in an aqueous dispersion of hybrid nanoparticles with diameters 85–180 nm (dispersed) or 20–50 nm (dried). The reaction conditions were optimized for different precursors by evaluating the relative amount ammonium hydroxide and maximizing the incorporated palm oil up to 70 wt.%. The interactions between palm oil and polymer phase have been studied by TEM, IR, Raman spectroscopy and thermal analysis (TGA, [TM] DSC). From Raman spectra, the amount of imide and reacted oil were quantified. Through concurring effects of imidization and coupling of fatty acids, the imidization needs a slight excess of NH3 relatively to maleic anhydride. The oxidative stability highly depends on oxidative crosslinking of free or non-reacted oil. Comparing the imide content from spectroscopic and thermal analysis suggests that a complex rigid imide phase without strong relaxation behavior has formed in combination with oil.

Performance of organic nanoparticle coatings for hydrophobization of hardwood surfaces

The protection of wood surfaces against water is a primary requirement to enhance their life-time and durability. In this article, a hydrophobic surface modification of selected hardwood surfaces (including high-density (HD) and low-density (LD) samples) is presented, by coating them with waterborne imidized nanoparticles under pure conditions or with vegetable oil. The performance of both nanoparticle coatings relative to noncoated and oil-coated samples was evaluated by water contact angles, microscopy, and optical profilometry. The pure nanoparticle coatings often increase the hydrophobicity, but they do not yet form a fully protective layer due to their porous structure after drying. The nanoparticle coatings with vegetable oil form a continuous layer with a maximum contact angle of 118°. The coating formation highly depends on the wood density (and resulting surface porosity), resulting in spreading of the aqueous dispersion on HD wood and penetration on LD wood. A thin continuous nanoparticle coating with incorporated vegetable oil provides highest contact angles, as the roughness of the original wood fibers remains visible in the surface profile.