Ion Irradiation on polytetrafluoroethylene(PTFE) has been carried out to improve adhesion to metal and to adhesive cement. Argon ion was irradiated on the polymer, and amount of Ar⁺ was changed from 10¹⁴ ions/cm² to l×10¹⁷ ions/cm² at 1 keV, and 4 ml/min of oxygen gas was flowed near the polymer surface during the ion irradiation. Wetting angle was changed from 100 degree to 70 - 150 degree depending on the ion beam condition. The changes of wetting angle and effects of Ar⁺ irradiation in oxygen environment were explained in a view of surface morphology due to the ion beam irradiation onto PTFE and formation of hydrophilic group due to a reaction between irradiated polymer chain and the blown oxygen. Strongly enhanced adhesions were explained by interlock mechanism, formation of electron acceptor groups on the modified PTFE, and interfacial chemical reaction between the irradiated surface and the deposited materials.

The strength of joints between Teflon FEP (Type A) and 500- to 1000-Å gold layers deposited by evaporation can be greatly increased if the Teflon surface is subjected to electron-beam bombardment prior to the evaporation process. Typically, joint strengths of about 60 kg/cm², approaching the bulk strength of Teflon, are obtained for treatments with electron-beam energies in the range of 5 to 20 keV and intercepted charge densities of about 5 X 10⁻⁶ C/cm². This compares with gold–Teflon joint strengths of about 10 kg/cm² for untreated material. The increase in joint strength is believed to be primarily due to crosslinking caused by the electron bombardment. Compared to the other known treatments to improve gold–Teflon joints, the present method has the advantage that the charge-storage properties of the Teflon are not irreversibly degraded. It is possible, for example, to store charge densities up to 3 X 10⁻⁸ C/cm², on 25-μm films treated with this method, with the same favorable charge-retention properties and thermally stimulated current characteristics as obtained for untreated Teflon.

The surface modification of synthetic fiber fabrics via corona discharge treatment and subsequent graft polymerization was investigated. Polyethylene (PE) nonwoven fabric and polyamide-6 (PA-6) nonwoven fabric were used as base fabrics. Acrylic acid (AAc) was graft polymerized onto the fabrics via corona discharge pre-treatment. The grafted amounts of PAAc were dependent on the grafting time, that of PA-6 being higher than that of PE. It was confirmed that the surface of the fibers constructing the fabric was fully covered with PAAc after the 20 min reaction. The surface of the PAAc grafted fabrics was characterized by X-ray photoelectron spectroscopy. The leakage of electrostatic charge from the fabric was determined and the dyeability was studied with methylene blue. The period of time in which the charge potential attenuated to 1/2 of the initial potential decreased drastically by grafting with PAAc. The grafted amount was enough for dyeing the entire fabrics.

Poly(vinylamine) (PVAm) was grafted on a poly(ethylene) (PE) film surface via the surface graft polymerization of N-vinylformamide (NVF) and N-vinylacetamide (NVA) and the subsequent hydrolysis of those grafted polymers. The surface was characterized by X-ray photoelectron spectroscopy (XPS), contact angle, moisture absorption, and the leakage of electrostatic charge from the films. PNVF and PNVA were introduced onto the surface of the PE film successfully, in spite of the fact that the initiator for polymerization was a peroxide group. The grafted amounts of PNVF and PNVA were dependent on the grafting time. A PVAm-grafted surface was obtained via the hydrolysis of the grafted PNVF. The grafted-PNVA was not hydrolyzed under mild hydrolysis. The obtained PVAm-grafted surface appeared to be useful for various applications, such as protein immobilization or chemical modification. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 1583–1587, 1999
https://onlinelibrary.wiley.com/doi/abs/10.1002/%28SICI%291097-4628%2819990620%2972%3A12%3C1583%3A%3AAID-APP11%3E3.0.CO%3B2-0

We present new spreading-drop data obtained over four orders of time and apply our new analysis tool G-Dyna to demonstrate the specific range over which the various models of dynamic wetting would seem to apply for our experimental system. We follow the contact angle and radius dynamics of four liquids on the smooth silica surface of silicon wafers or PET from the first milliseconds to several seconds. Analysis of the images allows us to make several hundred contact angle and droplet radius measurements with great accuracy. The G-Dyna software is then used to fit the data to the relevant theory (hydrodynamic, molecular-kinetic theory, Petrov and De Ruijter combined models, and Shikhmurzaev’s formula). The distributions, correlations, and average values of the free parameters are analyzed and it is shown that for the systems studied even with very good data and a robust fitting procedure, it may be difficult to make reliable claims as to the model which best describes results for a given system. This conclusions also suggests that claims based on smaller data sets and less stringent fitting procedures should be treated with caution.

High density polyethylene sheets 2 mm thick were flame treated to modify the surface properties. Sheets treated using a flame with air to gas (methane) ratio ∼ 10:1 at different distances between the inner cone tip of the flame and the polymer surface were investigated. Grafting of selected monomers as maleic anhydride, acrylamide and glycidyl methacrylate was attempted by flame treatment of sheets covered with a monomer layer. Good grafting results were obtained with acrylamide and maleic anhydride. The surface temperature-time dependence during the flame treatment was measured with a high resolution thermocouple. Scanning Electron Microscopy (SEM) allowed evidencing a modified thickness of about 120 μ. The chemical surface modification was studied by X ray Photoelectron Spectroscopy (XPS) and Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFT). The hydroxyl, carbonyl and carboxyl content was measured after derivatization with reagents containing an elemental tag to facilitate XPS analysis of surface functional groups. In comparison to the untreated polyethylene, wetting tension and contact angle of the flamed materials showed a strong variation. This variation was almost independent of the distance between the flame and the polymer surface. Adhesion between treated polyethylene and a polyurethane adhesive was determined using T-peel test measurements. High adhesion levels were found with flame treated polyethylene at 5 mm distance. XPS results indicate that when adhesion is high, the hydroxyl is in excess compared to the other measured functions, i.e. carbonyl and carboxyl species.

Earlier systematic studies of the angle of contact (9) exhibited by drops of liquid on plane, solid surfaces of low surface energy have revealed a regular linear variation in cos 9 with the surface tension (ylv) of a large variety of liquids; this led to the concept of the critical surface tension of spreading (yc) and its use in characterizing the wettability of organic solids and of high energy surfaces coated with adsorbed organic films. Effects of the nature and packing of the atoms or organic radicals in the organic surface in determining the wetting of the solid are summarized. Simple and useful correlations have been found between „and the constitution of low energy solid surfaces. It is concluded that usually atoms more than a few atom diameters below the surface have no influence on wetting. The “retraction method” of preparing monomolecular films from solutions on solids is shown to be a direct consequence of the above constitutive law of wetting. The same analysis can be applied to a pure liquid also, and it results in the explanation of the behavior of the autophobic liquids at room temperature and of the process of depositing a monolayer on a solid by retraction from the melt over a range of temperatures.

The equilibrium contact angle (θ_E) has been measured for some sixty diverse liquids with respect to a smooth platinum surface coated with an adsorbed oriented monolayer of n-octadecylamine. Linear relations were found between cosine θ_E and the liquid surface tension (gg_LV^∘) for every homologous series. When homology was disregarded, the cos θ_E-υs.-γ_LV^∘ data for all the liquids collected on three straight lines, two of which were approximately parallel. Simple curvilinear relations obtained between the work of adhesion (W_A) and γ_LV^∘ and between the final spreading coefficient (S_{LV^∘/SV^∘}) and γ_LV^∘, the constituents of each set of three curves being the same as before. The grouping onto multiple lines corresponds to differences in the solid/liquid interfacial tension, γ_SL, and to the relative solvent power of the liquids for the adsorbed octadecylamine. The same correlation obtained for the critical surface tension (γ_C), which was shown to be specific both to the homologous series and to the solid surface.

Constant values of the free energy decrease on immersion (f_SL) were observed for the homologous series of n-alkanes and n-alkyl ethers, while the alkylbenzene series showed a linear decrease with increasing γ_LV^∘. From the small range and low experimental values of f_SL observed for many unrelated liquids, it is concluded that the free surface energy of the monolayer-coated solid is probably not much more than 28.5 erg/cm.².

The striking similarity observed for the wetting properties of the monolayer-coated surfaces compared with those reported previously for surfaces of single crystals of n-hexatriacontane and bulk paraffin (5) demonstrates that the wetting behavior of a surface is essentially controlled by the nature and packing of the outermost group of atoms in the molecules. Intercomparison of wetting data for the monolayer with reference data obtained for a surface of methyl groups in closest packing (i.e., n-hexatriacontane single crystals) is proposed as an approach for determining, from contact angle measurements, the packing of adsorbed films at the solid/air interface.

A plot of the initial spreading pressures F sub ba or initial spreading coefficients S sub ba against the surface tensions of a homologous series of organic liquids b can be used to determine the critical surface tension for spreading on a second substrate liquid phase a. Straight-line relations are found for various homologous series. The intercept of that line with the axis of abscissas F sub ba 0, or S sub ba 0 defines a value of spreading for that series. This method is advantageous because it eliminates the need for measuring or calculating the contact angle of lens b floating on liquid a, it can be applied to any liquid substrate, and it is applicable even when spreading does not lie within the range of surface tensions of the members of the homologous series of liquids b. The value of spreading for the waterair interface was determined in this way using several homologous series of pure hydrocarbon liquids. The lowest value found was 21.7 dynescm at 20 deg C for the n-alkane series. Higher spreading values were obtained using olefins or aromatic hydrocarbons as the result of interaction between the unsaturated bond and the water surface. Since the results are analogous to those reported earlier for solid surfaces, it is concluded that the clean surface of water behaves as a low-energy surface with respect to low-polarity liquids. This result is to be expected if only dispersion forces are operative between each alkane liquid and water.

Many wetting processes involve interpretation using Young’s equation to describe contact angle equilibrium on a solid surface. By assuming the solid to be rigid, no account is made of the component of the liquid surface tension perpendicular to the solid surface. It is shown that a wetting ridge must be formed and, although negligible for hard solids, this mesoscopic disturbance of the solid near the wetting front can have significant consequences on a soft solid. The theory of triple line displacement, taking into account viscoelastic dissipation in the wetting ridge, is developed both for wetting and dewetting processes. Experimental studies using tricresyl phosphate and two types of model solid—a rigid polymer and silicone elastomers—have been carried out. Both for wetting and dewetting, triple line motion is markedly slowed down on the soft solids as a result of viscoelastic dissipation near the triple line. Theoretical predictions and experimental findings are found to be in good agreement.

Wetting hysteresis due to isolated surface heterogeneities is now fairly well understood but when the solid presents a population of defects, complex cooperative effects between neighbours may exist. One such effect is that of ‘shadowing’, in which a proportion of the flaws near the triple line, and which would otherwise contribute to hysteresis, are masked by already existing deformations to the wetting front caused by neighbouring heterogeneities. This renders them inactive and, as a result, the hysteretic wetting force is only expected to be a linear function of density for sparse populations. Theoretical predictions are compared with experimental results obtained with model heterogeneous surfaces consisting of overhead projector transparencies bestrewn with circular ink spots - the defects. Agreement is found to be satisfactory when intrinsic angles on both the homogeneous solid and the flaws are finite, whereas the concordance is less satisfactory when the contact angle of the liquid on the homogeneous solid is zero.

The equilibrium at the triple line where a liquid and a fluid (either vapour or a second liquid immiscible with the first) meet on a solid surface was originally described nearly two centuries ago¹. By using a simple vectorial argument, the well-known Young equation may be obtained by resolution of the three interfacial tensions, γ, parallel to the solid surface:

$${\gamma _{s2}} = {\gamma _{s1}} + {\gamma _{12}}\cos \theta$$
(1)

where 1, 2 and S represent respectively the liquid, the fluid and the solid and θ is the contact angle measured in phase 1. Nevertheless, an objection has on occasion been presented. Although everything is balanced parallel to the solid surface, nothing would seem to counteract the vertical component γ₁₂ sin θ.^2,3 When the solid is treated as perfectly rigid, it is possible to apply variational calculus and the criterion of minimum free energy at equilibrium. The result is that equation 1 is perfectly correct.^4–9 When the solid is considered to be elastic, but very thin, a variational treatment leads us to take into account, in addition to interfacial effects, those due to elastic strain energy and (implicitly) gravity¹⁰. The approach invokes the modelling of the solid either by thin plate or membrane theory. This treatment leads to modified equilibrium conditions although in practice the effect will be very small except for very thin solids (cell walls?).

In investigating the effect of the surface energetics of substrate materials on the adhesion characteristics of poly(p-xylylene) and poly(chloro-p-xylylene) by the “Scotch Tape” method, it was found that if the substrates had not been preconditioned (treated with argon or a methane plasma), the adhesion was poor. The characteristics of water resistant adhesion that were observed when coated substrates were boiled in 0.9% sodium chloride solution were found to vary from excellent (when the polymer did not peel from the substrate after three cycles of 8 hours of boiling and 16 hours at room temperature) to poor (when the polymer peeled off almost immediately). It was noticed that water resistant adhesion depends on the hydrophobicity of the substrate material (the greater the hydrophobicity, the greater the adhesion) and is not related to the dry adhesive strength of poly(p-xylylene). The oxygen glow discharge treatment of the substrates decreased both the dry and wet adhesive strength of the polymer. The effect of the argon glow discharge treatment depended on the surface energetics of the substrate, and the methane glow discharge treatment increased both the dry and wet adhesive strength of the polymer. These preconditioning processes are discussed in terms of the sputtering of the material from the wall of the reactor in contact with the plasma and the deposition of the plasma polymer of the sputtered material on the substrate surface.

This paper describes various aspects of water-based coatings and printing processes with special emphasis on the surface characteristics of coating/printing films. The film formation depends significantly on the surface properties of formulated coating/ink, and their interactions with substrates. Several surface parameters in relation to coating defects are briefly described. The mechanisms of printing processes and coating/ink film formation by water-based systems are presented. It has been shown that the formation of surface tension gradient during film curing determines the quality of the coating and printing films. Results demonstrate that the incorporation of suitable additives in the formulation can considerably minimize the crater formation. The hydrophilic-lipophilic balance (HLB) concept and the effect of surfactant concentration on pigment dispersion in an aqueous medium are discussed.

The magnitude of the hydrophobic effect, as measured from the surface area dependence of the solubilities of hydrocarbons in water, is generally thought to be about 25 calories per mole per square angstrom (cal mol^-1 Å^-2). However, the surface tension at a hydrocarbon-water interface, which is a "macroscopic" measure of the hydrophobic effect, is ≈72 cal mol^-1 Å^-2. In an attempt to reconcile these values, alkane solubility data have been reevaluated to account for solute-solvent size differences, leading to a revised "microscopic" hydrophobic effect of 47 cal mol^-1 Å^-2. This value, when used in a simple geometric model for the curvature dependence of the hydrophobic effect, predicts a macroscopic alkane-water surface tension that is close to the macroscopic value.

Certain aspects of the adsorption theory of adhesion are developed more fully than has been done previously. The consequences of nonreciprocity of spreading are pointed out, and are used to develop a more general practical point of view with respect to the adhesive bonding of materials of low-surface free energy. The system epoxy adhesive-(nonsurface-treated) polyethylene, normally considered nonadherent, is investigated experimentally in some detail. It is shown how this system, without material modification, can be made adherent. An area of study for possible adhesives for materials of lowsurface free energy is suggested.

In conventional offset lithographic printing, it has been well established that the existence of a continuous layer of fountain solution (FS) on the surface of the non-image area is an essential condition to ensure correct operation of lithography. However, the mechanistic function of FS in preventing the ink from being transferred onto the non-image area has not been fully understood. Several major mechanistic interpretations can be found in the literature, which are based either on comparing of static works of adhesion and cohesion of ink and FS, or on the splitting of the 'weaker' FS layer. Although the latter becomes more accepted, direct experimental evidence is difficult to find in the literature. On the other hand, confusing information found in the literature showed that the ink-transfer (or non-transfer) observations reported in many case studies correlate well with simple comparisons of works of adhesion, cohesion and spreading data of ink/FS, ink/plate and FS/plate obtained under the static condition. These results, therefore, imply that, in explaining the function of FS in preventing ink transfer to the non-image area, the ink/FS interfacial adhesion failure would be the dominant mechanism. The work presented in this study covered two specific areas in order to address and better understand the responses of ink and FS layers and their interface to forces encountered during ink transfer. Firstly, an analysis of lithographic plates contaminated with a cationic polymer revealed that the violation of the ink non-transfer condition of the plate non-image area due to contamination could be predicted by traditional criteria of plate wetting and works of adhesion and cohesion. However, these traditional criteria cannot reliably predict the non-transfer condition of the ink on the clean non-image area that was covered by FS. Secondly, in some novel experiments conducted in this study using ice or Teflon as a substrate, the works of adhesion and cohesion were not able to predict ink transfer in most cases. Direct experimental evidence from this work revealed that splitting of the FS layer was involved in the prevention of ink transfer to the non-image areas, and that the thickness of the FS layer was critical in allowing the splitting to occur.

The effects of flame treatment on the surfaces of a propylene-ethylene copolymer have been studied using XPS, contact angle measurement, vapour-phase derivatization and an adhesion test. The results obtained were compared to those from the homopolymer. An optimum air-to-gas ratios of ∼11:1 has been found. Close correspondence between water contact angle and oxygen concentration was found, with the exception of high oxygen concentrations. The orientation or migration of functional groups away from the surface has been proposed to cause the non-correspondence between water contact angle and oxygen concentration. Diiodomethane advancing contact angle was found to remain constant, independent of flame conditions. XPS analysis in conjunction with vapourphase derivatization with trifluroacetic anhydride (TFAA) suggests that up to 20% and 30% of the oxygen introduced in the surfaces is present as hydroxyl groups for propylene homopolymer and the copolymer, respectively. High adhesion levels of the flame-treated copolymer with a polyurethane-based paint were found. In most cases, the adhesion failure was complex, but involved the cohesive failure of the copolymer.

X-ray photoelectron spectroscopy (XPS) has been used to study the effects of flame treatment on three propylene polymers, i.e. a homopolymer, an ethylene-propylene copolymer and a rubber-modified polypropylene. Angle-resolved XPS has shown an enrichment in oxygen concentration at the near surface for all three propylene polymers when treated with a mild flame. A depletion in oxygen has been shown at the near surface of the rubber-modified polypropylene treated with an intense flame. The use of simple surface composition models shows that the oxidation depth induced by a mild flame treatment is around 50 Å, and that oxygen-containing functional groups may have reoriented or migrated a few ångströms away from the near surface of the rubber-modified polypropylene during the treatment with an intense flame.

We demonstrate that stable microwave-coupled atmospheric pressure nonequilibrium plasmas (APNEPs) can be formed under a wide variety of gas and flow-rate conditions. Furthermore, these plasmas can be effectively used to remove surface contamination and chemically modify polymer surfaces. These chemical changes, generally oxidation and crosslinking, enhance the surface properties of the materials such as surface energy. Comparisons between vacuum plasma and atmospheric plasma treatment strongly indicate that much of the vacuum-plasma literature is pertinent to APNEP, thereby providing assistance with understanding the nature of APNEP-induced reactions. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 95–109, 2002
https://onlinelibrary.wiley.com/doi/abs/10.1002/pola.10056

An atmospheric pressure non-equilibrium plasma (APNEP) developed in the UK by EA Technology Ltd is currently being investigated in collaboration with the University of Surrey. Of the many applications of surface modification that can be induced using plasmas, adhesion enhancement is one of the most commercially important. In this paper, we illustrate the use of an atmospheric plasma to enhance the adhesion characteristics of low-density polyethylene (LDPE) and poly(ethylene terephthalate) (PET). The polymers were treated in the remote afterglow region of an atmospheric pressure plasma to avoid the thermal effects that can cause degradation for thermally sensitive materials when placed in direct contact with the plasma. Reactive (oxygen containing) and inert (oxygen free) atmospheric plasmas rapidly impart adhesion enhancement by a factor of two to ten as measured by 180° peel tests. However, extended exposure to the atmospheric plasma does not impart additional adhesion enhancement as the surface is ablated revealing the underlying polymer with poor adhesive characteristics. In contrast, vacuum plasma treated LDPE and PET show increased adhesion with extended plasma treatment. An adhesion enhancement in excess of two to three orders of magnitude was found to be achievable for vacuum plasma treatment times greater than 10 min.

Atmospheric pressure non-equilibrium plasma (APNEP) has been developed in the UK by EA Technology Ltd and is currently being investigated in a joint project with the University of Surrey. APNEP has been used to induce surface modification changes on commodity polymers such as high density polyethylene (HDPE), low density polyethylene (LDPE), polypropylene (PP), poly(ethylene terephthalate) (PET) and poly(methyl methacrylate) (PMMA).

A stable atmospheric pressure glow discharge can be formed with a variety of gases, (e.g., nitrogen, air, argon and helium). In all cases, the plasmas are capable of inducing surface modification of commodity polymers in the near-field and remote afterglow regions. However, as APNEP can have a significant thermal component, care must be taken to avoid thermal decomposition of the polymers.

This study has used differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) to investigate the thermally induced transitions and thermal decomposition behaviour of commercial polymers. The DSC measurements give melting points, heats of fusion and crystallinities. TGA has been used to measure the onset of thermal degradation in both air and nitrogen atmospheres. In parallel with these experiments, temperature profiles of the downstream region of APNEP have been recorded. As a result, positioning of samples and residence times to avoid thermal damage to the substrates can now be achieved.

An atmospheric pressure non-equilibrium plasma (APNEP) has been developed in the UK by EA Technology Ltd and is currently being investigated in collaboration with the University of Surrey. The main focus is the use of atmospheric pressure plasmas to modify the surfaces of commercially important polymers including polyolefins, poly(ethylene terephthalate) and poly(methyl methacrylate). These surface modifications include surface cleaning and degreasing, oxidation, reduction, grafting, cross-linking (carbonization), etching and deposition. When trying to achieve targeted surface engineering, it is vital to gain an understanding of the mechanisms that cause these effects, for example, surface functionalization, adhesion promotion or multi-layer deposition. Hence comparisons between vacuum plasma treated surfaces have also been sought with a view to using the extensive vacuum plasma literature to gain further insight. In this paper, we will introduce the APNEP and compare the key characteristics of the plasma with those of traditional vacuum plasma systems before highlighting some of the surface modifications that can be achieved by using atmospheric plasma. Data from the analysis of treated polymers (by spectroscopy, microscopy and surface energy studies) and from direct measurements of the plasma and afterglow will be presented. Finally, our current understanding of the processes involved will be given, particularly those that are important in downstream surface treatments which take place remote from the plasma source.