The generation mechanism of demolding force based on the mold-part interface contact mode in micro-injection molding

Demolding is the key step of micro-injection molding (μIM), which influences the molding quality of polymer parts to a great extent. The essence of demolding is to overcome interface interactions between the mold cavity surface and the part surface, which is closely related to factors such as the cavity surface topography and process parameters. However, a theoretical model predicting the demolding force of μIM is still lacking, resulting in the difficulty of improving the quality of demolding in μIM. Therefore, a comprehensive demolding force model is proposed to clarify the demolding mechanism of μIM. The model focuses on the adhesion behavior and friction behavior in the special contact mode formed by the replication effect of μIM. The theory proof and computation results indicate that the cavity surface topography determines the contact mode of the mold-part interface (MPI), which in turn changes the real interface contact area, the composition mode of adhesion stress, and the friction forms, thereby affecting the adhesion and friction force during demolding. In addition, the process parameters can also change the contact mode of MPI as the process parameters have a great effect on the filling capacity and the degree of replication, thereby influencing the demolding process.     

Tribological properties of ethylene–propylene–diene rubber + polypropylene + thermal‐shock‐resistant ceramic composites

This work is part of a program on composites used in thermoelectric devices. Tribological properties of dynamic vulcanizate blends of polypropylene and ethylene‐propylene‐diene rubber filled with 5 wt% of microscale powder have been studied. The microscale thermal‐shock‐resistant ceramic filler contains α‐Al₂O₃, mullite (3Al₂O₃ · 2SiO₂ or 2Al₂O₃SiO₂), β‐spodumene glass‐ceramic and aluminium titanate. We found that our ceramic particles are abrasive; they cause strong abrasion of softer steel ball surfaces during dry sliding friction. To overcome the difficulty of particle dispersion and adhesion, the filler was modified through grafting using three types of organic molecules. Dry sliding friction was measured using four types of counter‐surfaces: tungsten carbide, Si₃N₂, 302 steel and 440 steel. Thermoplastic vulcanizate filled with neat ceramic powder shows the lowest friction compared to composites containing the same but surface‐treated powder. We introduce a ‘bump’ model to explain the tribological responses of our composites. ‘Naked’ or untreated ceramic particles protrude from the polymer surface and cause a decrease of the contact area compared to neat polymer. The ball partner surface has only a small contact area with the bumps. As contact surface area decreases, so does friction and the amount of heat generated during sliding friction testing. Chemical coupling of the ceramic to the matrix smoothens the bumps and increases the contact surface, giving a parallel increase in friction. Copyright © 2012 Society of Chemical Industry     

Prevention of Ice Accretion on Aluminum Surfaces by Enhancing Their Hydrophobic Properties

The accretion of ice on the surfaces of power network systems, aircraft, communication networks, etc., is known to cause serious problems that often lead to costly safety issues. An ideal solution would be to prevent ice from accumulating in the first place, rather than waiting for ice to accrete and then to de-ice which is both time-consuming and expensive. This may be accomplished by depositing coating materials that are icephobic. A low dielectric constant surface is expected to reduce the adhesion of ice due to the screening of mirror charges, thereby eliminating one of the strongest interaction forces — the electrostatic force of attraction — at the ice–surface interface. Superhydrophobic surfaces, which demonstrate high water-repellency due to the negligible contact area of water with these surfaces, are also expected to minimize the contact area of ice. In the present research work, both concepts were studied by producing superhydrophobic nanorough low-ε (dielectric) surfaces on aluminum. Superhydrophobic properties were achieved on surfaces of aluminum by creating a certain nanoroughness using a chemical etch followed by 'passivation' of the surface by a low surface energy coating of rf-sputtered Teflon, providing a water contact angle greater than 160°. The same behavior is reported even when the nanorough substrates were coated with dielectric thin films of ZnO (lower ε) or TiO 2 (higher ε) prior to passivation. It is found that the superhydrophobic nanorough low energy surfaces are also icephobic and the presence of a low dielectric constant surface coating of Teflon (ε = 2) allows a considerable reduction of the ice adhesion strength. Ice adhesion strengths were determined using a centrifugal ice adhesion test apparatus.     

Surface modification of polymers and practical adhesion

The, nature of polymer surfaces has received increasing attention as the use of these materials, in a variety of forms, increases yearly. Modifications of polymer surfaces for adhesion, friction, and diffusion oriented appiications have necessitated a careful analysis of the surfade region morphology (surface physics) and chemical properties of the surface layer (surface chemistry). The behavior of composite structures has involved the discipline of classical fracture mechanics. The orientation of polymeric species or additives which migrate to the interface may modify the wetting characteristics and, most certainly, the frictional properties in addition to the diffusion of penetrant species beyond the boundary layer. The above topics are discussed within the framework of recent analytical and theoretical developments in surface science. The findings of these recent studies have facilitated many exciting technological advances.     

Thermal and spectroscopic analysis of worn polyoxymethylene surfaces and wear debris explaining degradation and polymerisation mechanisms

Degradation and polymerization of polyoxymethylene homopolymer (POM-H) surfaces after sliding at 8 to 150 MPa and 0.005 m/s over a total sliding distance of 3000 m is investigated by using thermal analysis (DSC, TGA, DTA) and Raman spectroscopy of worn surfaces or wear debris. There is mainly mechanical interaction and slight softening at 8 MPa (relatively high friction, low wear), softening at 16 to 55 MPa (decreasing friction and high wear) and finally melting at 150 MPa (very low friction, overload wear). At low contact pressures, wear debris remains amorphous and degradation of noncrystallised material during sliding manifests in broadening of the melting peak below the melting temperature. Degradation of C–O–C due to chain scission and radical reactions into CH₃ end groups are illustrated by Raman spectra. It is confirmed that the debris has long resident times and the maximum polymer surface temperature (T* = 93°C) is below the crystallisation temperature. At intermediate contact pressures, crystallisation results in a polymer fraction with higher thermal resistance. From the calculated temperatures T* = 120 to 150°C, crystallisation is beneficial for coherent transfer with larger particle sizes. At high contact pressures, the wear debris is immediately removed from the contact interface due to melting (T* = 200°C) and has thermal properties similar to the bulk material. There is no reaction between the debris in the interface, resulting in a thick polymer transfer film.     

Adhesion and friction studies on silicon dioxide nanoparticle-textured surfaces prepared by the spin-coating process

Silicon dioxide nanoparticle-textured surfaces were prepared by the spin-coating process. The adhesion and friction properties of the nanoparticle-textured surfaces were investigated using an atomic force microscope colloidal probe. Experimental results revealed that the nanoparticle-textured surfaces can significantly reduce adhesive and friction forces compared with a flat surface. The main reason for this phenomenon was that the nanotexture can reduce contact area between the sample surface and the colloidal probe. The relationships between surface root mean square (RMS) roughness, packing density, and spinning rate were also discussed. The effects of surface RMS roughness and packing density on the adhesion and friction behaviors of the nanotextured surfaces were investigated. The adhesive and friction forces of the nanoparticle-textured surfaces decreased with increasing packing density. The friction forces of the nanoparticle-textured surfaces increased with increasing applied load and sliding velocity. This approach should be applied to new developments in nanosystems to reduce adhesive and friction forces between contact pairs.     

Plasma treatment of wood–polymer composites: A comparison of three different discharge types and their effect on surface properties

Three different discharge types, based on the principle of a dielectric barrier discharge at atmospheric pressure, were investigated with regard to their influence on the adhesion properties of a series of wood–polymer composites. Wood flour (Picea abies L.) filled polypropylene and various proportions of polyethylene were manufactured either through extrusion or injection molding. The composites’ surfaces were activated by coplanar surface barrier discharge, remote plasma, and direct dielectric barrier discharge. The changes in wettability due to the pretreatment were investigated by contact angle measurement using the sessile drop method and calculation of surface free energy (SFE). It could be shown that wettability was improved by all three types of discharge, the contact angle decreased and the SFE correspondingly increased. X‐ray photoelectron spectroscopy revealed an increase in the O/C ratio at the material's surface. An improvement in coating adhesion was demonstrated by crosscut and pulloff tests. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43376.     

Frictional characteristics of stiff,high aspect ratio microfiber arrays based on cyclic olefin polymers

Cyclic olefin polymer (COP) microfiber arrays with diameters in the range of 0.6–5 μm and up to ~20 μm in length are successfully fabricated using polycarbonate membranes as templates, and their macroscale friction properties are evaluated against smooth glass surfaces. Increasing the aspect ratio of the fibers decreases the effective modulus and increases the effective work of adhesion, which favor a better compliance, resulting in higher friction forces. On the other hand, the shape of the fibers as well as inter-fiber adhesion at high aspect ratio complicates the dependence of friction on fiber geometry. Among the arrays investigated, the 2 and 0.6 μm diameter arrays exhibited excellent friction performance (up to ~6 and 9 N/cm², respectively) and high fractional contact area (50–60%). These results suggest that arrays with high friction properties can be easily obtained by molding COPs from polycarbonate membranes.     

Limit Cycles in Dynamic Adhesion and Friction Processes: A Discussion

Simple equations exist relating adhesion and friction forces. These apply to simple attachment-detachment processes and steady-state (smooth) sliding conditions. However, in the case of more complex, such as polymer, surfaces both the adhesion and friction can be very complex, irreversible, and nonlinear, exhibiting stringing and tack in the former and stick-slip sliding in the latter. We explore possible relationships between such nonlinear adhesion and friction processes. Based on recent experiments we find that certain types of “limit cycles,” relating the (normal) adhesion and (lateral) friction forces, F _⊥ and F _∥, to the relative velocities, V _⊥ and V _∥, of the surfaces during an attachment-detachment process or stick-slip sliding, bear a very similar resemblance to each other. We briefly discuss the theoretical and practical implications of describing such dynamic processes in terms of limit cycles.     

Effects of facile amine‐functionalization on the physical properties of epoxy/graphene nanoplatelets nanocomposites

Graphene nanoplatelets (GNPs) have excellent thermal, electrical, and mechanical properties. The incorporation of GNPs into a polymer can remarkably enhance the thermal and mechanical properties of the polymer especially when GNPs are well dispersed in the polymer matrix with strong interfacial bonding. Therefore, in this study, GNPs were amine‐functionalized by covalently bonding 4,4′‐methylene dianiline onto their surfaces via a facile synthetic route. The amine‐functionalization was confirmed by FTIR spectroscopy and TGA. Epoxy/GNPs nanocomposites were prepared and their curing behavior, thermomechanical properties and impact strength were investigated. The amine‐functionalization increased curing rate, storage modulus, thermal dimensional stability, and impact strength of the nanocomposites. The SEM images for the fracture surface of the nanocomposite with amine‐functionalized GNPs showed a smooth and ductile failure‐like surface, resulted from the improved interfacial bonding between GNPs and the epoxy matrix. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42269.     

Underwater Adhesion Measurements using the JKR Technique

The JKR (Johnson–Kendall–Roberts) method of contact mechanics has been widely utilized for measuring adhesion properties between a deformable elastomeric lens and various materials. Such measurements are normally performed in air. We attempted to verify whether the JKR technique could be practical for evaluating adhesion properties under water. After modifying the common JKR apparatus to be suitable for underwater studies, two types of hydrophobic coating systems, silicone/silicone and silicone/silanized silicon wafer, were used. The work of adhesion (W _A ) values obtained from loading measurements and under zero load were found to be slightly smaller than the values estimated using surface energies and contact angles of water formed on the surfaces of these coatings. One possible cause for the slightly smaller values could be contamination/alteration of the coating surface properties upon immersion in water. The results suggested that, with proper control of experimental conditions, the JKR technique could be extended to evaluate adhesion properties under water.     

Adhesion Frequency in Interfacial Dynamics of Copolymers

This paper introduces the concept of adhesion frequency that characterizes the dynamic interaction between a polymer matrix and a solid surface. It is argued that the pressure sensitive bonding of a polymer interface occurs within a critical time scale below which no wetting is established upon contact. The adhesion frequency can be viewed as the inverse of this critical time scale. The bonding kinetics is used to model transient adhesion and take into account the growth of bonding energy with time. The mechanics of the interfacial dynamics is used to derive expressions for the adhesion frequency when inertial, viscous or elastic effects dominate the surface deformation. Transient adhesion effects are also investigated experimentally using non-axisymmetric rotating bits and commercially-available copolymer adhesives. The rotating bits induce an oscillatory motion on the polymer surface. Experiments are conducted with the rotating bits spinning at a wide range of speeds making it possible to obtain the dynamic response of the polymer interface as well as to measure the adhesion frequency for the tested polymers.     

Geckos’ foot hair structure and their ability to hang from rough surfaces and move quickly

Geckos generate the necessary adhesion force through their foot hair. The direction of the gecko's foot hair is not perpendicular to its finger surface, giving compliance to the hair. The effect of this compliance on the adhesion force is analysed and expressed theoretically in terms of contact mechanics. We conclude that the compliance of the foot hair is sufficient to generate the large adhesion force necessary for adhesion to rough surfaces, and that the structure of the seta with the spatulae allow the normal adhesion force to be controlled, allowing the gecko to make quick steps.     

Investigation on pull-out behavior and interface critical parameters of polymer fibers embedded in concrete and their correlation with particular fiber properties

A crucial problem in concrete engineering is the corrosion of steel reinforcements. Polymer fibers as alternative reinforcement material can prevent corrosion; however, high adhesion to concrete and good fiber mechanics are necessary for polymers to be considered as an alternative reinforcement. This study tested different thermoplastic polymer materials to evaluate their level of adhesion to concrete. The adhesion properties of different self-drawn polymer fibers were analyzed by extracting the fibers from concrete using single fiber pull-out test (SFPT). To determine the adhesion mechanism, different polymer properties were analyzed and correlated to SFPT. Strong evidence was found that the fibers mechanical properties correlate with SFPT. Roughening the fiber surface increases the SFPT results significantly. While highly polar materials can support the adhesion process, a clear correlation could not be found. This study identifies high stiffness and roughness as the crucial properties of polymer fibers used in concrete engineering. If these factors can be engineered into the fiber, polymer fibers can present an alternative to steel in concrete reinforcement.     

Simulation of the attachment system with various tip shapes contacting rough surface

In this paper, we investigate the effects of various contact shapes on the adhesion performance for the attachment system contacting rough surfaces. Four different tip shapes of sphere, flat punch, torus and tape are considered. We demonstrate the effect of the tip size and the elastic modulus on the adhesion force through the adhesion analysis for single tips with four different shapes. In order to investigate an effect of tip shape on adhesion performance for the attachment system contacting rough surface with different σ values, single level attachment system was simulated. The attachment system is modeled from the geometrical size of spatulae of Tokay gecko. The effect of tip shape on the adhesion enhancement and how far its effect can be reached for the surface roughness are investigated. It is shown that the effect of tip shape on the adhesion enhancement is limited to the surface roughness with smaller than several micrometers and further adhesion enhancement can not be expected at rougher surfaces.     

Hybrid polymer matrix composite (UHMWPE-HPMC) as solid particle erosion resistant coating for sharp pipe elbows using numerical simulation and experimental analysis

A hybrid polymer matrix composite coating, resistant to solid particle erosion inside sharp elbows, consisting interlocking chains of molecules with the ability to deflect the surface impact stress and to uniformly distribute stresses along the hard-ceramic reinforcement mixture surface was developed. Formulated mixture of ceramic reinforcement particles mixtures (alumina, tungsten carbide, and silicon carbide) with polymer coupling agents; to increase adhesion to the metal surface, led to 600–700 HVN in ternary and 500–550 HVN in binary mixtures. This behavior coincides with high shear strength of 70–76 MPa, Young's and shear modulus of 8.86 and 13.4 GPa in ternary 15%Al₂O₃-5%WC-10%SiC, respectively. The low erosion weight loss of 0.1% and small coefficient of friction near 0.18 indicates the significant wear resistance of the ternary sample. The electron microscopic micrographs determined the dense smooth coating surfaces with adhesive interfaces with the substrate.     

Relating the Stickiness Property of Foods Undergoing Drying and Dried Products to their Surface Energetics

Stickiness is a common problem encountered in food handling and processing, and also during consumption. Stickiness is observed as adhesion of the food to processing equipment surfaces or cohesion within the food particulate or mass. An important operation where this undesirable behavior of food is manifested is drying. This occurs particularly during drying of high-sugar and high-fat foods. To date, the stickiness of foods during drying or dried powder has been investigated in relation to their viscous and glass transition properties. The importance of contact surface energy of the equipment has been ignored in many analyses, despite the fact that some drying operations have reported using low-energy contact surfaces in drying equipment to avoid the problems caused by stickiness. This review discusses the fundamentals of adhesion and cohesion mechanisms and relates these phenomena to drying and dried products.     

A study on the tribological behavior of polyethylene. II. Effects of operating conditions on the friction behavior of polyethylene

In this article, the effects of the operating conditions, i.e., load, oscillation speed, temperature, and contact modes on the friction behavior of polyethylene were studied through the SRV vibration friction test machine, the MHK-500 friction and wear test machine, as well as an on-line temperature testing device. The experimental results showed that the friction coefficient μ of polyethylene increases with increase of the oscillation frequency and amplitude, the speed, and PV value, while load has a quite complex impact on μ; suitable choice of load could reduce μ and smoothen the friction process. Contact modes of friction pairs have considerable effects on μ, because all the real contact area of the friction assembly, the pressure, the indentation of surface asperities, as well as the temperature rise and distribution in the contact region are related to contact modes. Temperature is a key factor determining the viscoelastic properties of polyethylene, and therefore has great effect on μ. On-line temperature testing offers a way to reveal the relations between temperature and the friction behavior of polyethylene. All the results obtained provide the basic data for establishing mathematical models and computational simulation methods to describe and study the tribological behavior of some polymer materials. © 1995 John Wiley & Sons, Inc.     

A new technique for evaluating ink–cellulose interactions: initial studies of the influence of surface energy and surface roughness

Ink–cellulose interactions were evaluated using a new technique in which the adhesion properties between ink and cellulose were directly measured using a Micro-Adhesion Measurement Apparatus (MAMA). The adhesion properties determined with MAMA were used to estimate the total energy release upon separating ink from cellulose in water. The total energy release was calculated from interfacial energies determined via contact angle measurements and the Lifshitz–van der Waals/acid–base approach. Both methods indicated spontaneous ink release from model cellulose surfaces, although the absolute values differed because of differences in measuring techniques and different ways of evaluation. MAMA measured the dry adhesion between ink and cellulose, whereas the interfacial energies were determined for wet surfaces. The total energy release was linked to ink detachment from model cellulose surfaces, determined using the impinging jet cell. The influences of surface energy and surface roughness were also investigated. Increasing the surface roughness or decreasing the surface energy decreased the ink detachment due to differences in the molecular contact area and differences in the adhesiom properties.     

Carbon fibers and composites with epoxy resins: Topography, fractography and interphases

Interphases exist in hybrid materials and significantly influence their mechanical performance. To find a bridge between the microscopic and macroscopic mechanical properties, this work investigates the nanoscopic nature of surface/interphases in terms of topography, fractography, adhesion and stiffness. Here, we show that variations in both adhesive and attractive forces on oxidized high modulus (HM) and intermediate modulus (IM) carbon fiber surfaces appear to result from the coating layer. The coating layer is critical for adhesive interaction with two different epoxy resins. The HM fiber has the apparently higher roughness but lower surface area than the IM fiber on the scanning scale of 200 nm. The surface roughness on a few tens of nanometer scale has no significant contribution to interphase adhesion from ‘mechanical interlocking’. In contrast, the true contact area on the nanometer scale plays a dominant role in interfacial adhesion. Using force volume nanoindentation, the stiffness of the resin region near the finished fiber surface was found to not depend on the distance from the fiber surface. Our observations suggest an energy-geometry link between critical interphase energy release rate by micromechanical testing and detailed fracture surface features.