This paper aims to provide an exhaustive and comprehensive overview on flame treatment as a valuable technique for improving the surface properties of polymers, especially polyolefins. It starts with a brief historical excursus on the origin of flame treatment, and the second section deals with the major fundamentals of flame chemistry, with a special focus on the combustion process and mechanism of surface activation. The most important parameters influencing the extent of the oxidation reaction along with relevant practical notes are discussed in the third section. The concluding section outlines how the most significant features of flame treatment can be profitably used to improve the wettability and adhesion properties of polyolefin surfaces, especially from the perspective of developing novel composite solutions such as polyolefins/bio-based coating pairs intended for many different applications.

The surface functionalization of polyimide (Matrimid^® 5218) films was carried out by cold plasma treatment with CF₄, N₂ and O₂ gases using a radio frequency discharge and the optimum plasma conditions were evaluated by water contact angle measurements. The surface hydrophobicity of polyimide films was obtained after CF₄ plasma treatment, while O₂ and N₂ plasma treatments contributed to the hydrophilic surface functionalization. X-ray photoelectron spectroscopy (XPS) results revealed the presence of CF_x, amino or oxygen-containing groups attached to the polyimide film surface depending on the treatment gas. A strong influence of the used plasma gas on the film roughness was determined by atomic force microscopy (AFM) measurements. The influence of the surface modification on CO₂, N₂ and O₂ gas permeation through the plasma treated films was evaluated. The permeation behaviour was characterized in terms of transport parameters, namely, coefficients of permeability, diffusion and solubility. The permeability coefficient of all plasma treated polyimide films for the studied gases (CO₂, N₂ and O₂) was found to decrease following the order of increasing the kinetic molecular diameter of the penetrant gas. Besides, the selectivity coefficient was found to be significantly increased after the plasma treatments – α_CO2/N2 was increased up to 36% and 98% for O₂ and N₂ plasma treated Matrimid^® 5218 films, respectively. The relationship between the gas permeation behaviour and the surface modification of polymer film by cold plasma was discussed.

Different plasma treatments (NH3, O2) were carried out on polytetrafluoroethylene (PTFE) for grafting polar groups and obtaining a stable, permanent hydrophilic surface. Plasma pretreatments (H2 and Ar) were also utilized to limit the aging, including the hydrophobic recovery, of the treated surface with time. Dynamic water contact-angle (WCA) measurements and X-ray photoelectron spectroscopy (XPS) analyses were performed to study in depth the chemical compositional changes as a function of ageing time. This paper illustrates mainly the remarkable effect of combining H2 plasma pretreatments with low-power NH3 plasma treatments for obtaining stable PTFE surfaces grafted with polar groups that exhibit permanent wettability. The results were expressed in terms of the fractions of mobile and immobile polar grafted groups.

Low-pressure plasma-treatments aimed to selectively graft-NH2 groups onto the surface of conventional polymers such as polyethylene, polystyrene and polyethyleneterephtalate are described. The combined use of plasma and surface diagnostics allowed elucidation of the effect of the experimental parameters on the extent of the surface modifications and to understand chemical mechanisms involved in the surface processes. The diagnostic approach is essential for engineering polymer surfaces with a dosed relative density of-NH2 groups, for scale-up and process transfer.

Careful measurements of the surface tension of solid polymers, y_s°, based on the data on contact angles for wetting liquids with various surface tension, y_L°, allows one to establish the functional dependence of y_s° = f(y_L°). This dependence is divided into three zones: one zone, where there is no dependence of y_s° on y_L° and two zones where y_s° changes linearly with y_L°.

The reasons for the appearance of the hysteresis of wetting are considered. The model is proposed according to which the hysteresis is the result of the orientations of molecules of wetting liquids which is preserved due to the action of surface forces even after the flow ceases.

The hydrophobic characteristic of polymers is considered a limiting property for its applications. To some extent, this has been overcome by techniques such as non-thermal plasma, which, even with a few seconds of application, can increase the surface energy and hydrophilic character of polymers. However, this technique is associated with advantages and disadvantages. Surface degradation related to oxidation and crosslinking are considered irreversible changes, in most cases, while the hydrophobic character is quickly restored, presenting a challenge to researchers all over the world. As a reversible behavior, efforts have been made to understand this particular characteristic of the hydrophobic recovery (or the aging effect) of polymers. The application of non-thermal plasma on polymeric surfaces has also been used in biomedicine as a sterilization device to control the growth of biofilms, as well as to increase the biocompatibility of prosthetic surfaces. This chapter discusses some particular characteristics of polyolefins exposed to plasma.

This work reports a methodology to improve the adhesion between poly(ethylene terephthalate) (PET) fibers and poly(hydroxyethyl methacrylate) (pHEMA) hydrogels by treating PET with ozone. The surface chemistry of PET was examined by water contact angle measurements, X-ray photoelectron spectroscopy (XPS), infrared reflection absorption spectroscopy (IRAS) and attenuated total reflectance infrared spectroscopy (ATR-IR) yielding information about the chemical functionalities at depths upon 0.6 μm. Ozone treatment introduces several polar groups in the surface of PET through oxidation and chain scission resulting in increased wettability. These groups include mostly carboxylic and anhydride groups and in small extent hydroxyl groups. Atomic force microscopy (AFM) analysis shows that the surface of ozone-treated PET films is fully covered with spherical particles that are removed after washing the film with water. During the washing step carboxylic functionalities were removed preferentially, as demonstrated by XPS and IR analysis. According to pull-out tests, PET monofilaments and bundles treated by ozone had a higher adhesion to pHEMA hydrogels than untreated ones. The apparent interfacial shear strength is 65% higher on pHEMA hydrogel containing an ozonated than an untreated PET monofilament. In addition, the force to pull-out an ozone-treated PET bundle from pHEMA hydrogel is ca. 81% higher than the one observed for the untreated bundle.

Air plasma treatment at low pressure was applied to modify the surface of a cellulose film with the aim to improve its wettability, dyeability and adhesion properties. The contact angles of different polar liquids on the treated films show an exponential decay with treatment time at a given power; the power–time reciprocity is followed. The calculated surface tension values exponentially rise to the same maximum value with a decrease of the polar fraction. ATR-FTIR analyses suggest that a cellulose dehydration takes place rather than a surface oxidation. The plasma treatment improves also the cellophane dyeability with typical dyes for cellulose fibers: the results of dye uptake follow the same trend as the surface energy. The bond strength of lap joints of cellophane with LLDPE film shows a strong improvement of the adhesion depending on the duration and the power of treatment. The whole results are consistent with ablation effects like those observed with air corona treatment rather than oxygen plasma.

Adhesion involves a detailed understanding of polymer synthesis and characterization, mechanics, and surfaces. This chapter reviews surface analysis and interphase analysis emphasizing polymer/metal systems. The interphase is a thin region between the bulk adherend and the bulk adhesive, as depicted in Figure 1. A surface oxide, either native or one produced by pre-treatment, is present on most metal adherends. A primer is often applied in a production process after pretreatment and before the application of an adhesive. Typical thicknesses for the oxide are 0.003–0.4 µm, for the primer 4 µm (0.16 mil), and for the adhesive 40 µm (1.6 mil). The interphase region is expected to have mechanical properties different from either the adherend or the adhesive. Measurement of these properties is important in understanding adhesion, for example, poorly durable bonds are often a consequence of poor interphase properties.(1,2) Thus, one of the frontier areas in adhesion science today is determining interphase properties.

This work demonstrates the usefulness of flow microcalorimetry for surface characterization of metal foils (aluminum) and polymer [poly (ethylene-co-acrylic acid)] fibers. It shows that the polymer to aluminum adhesion is dominated by Lewis acid/Lewis base type interactions. These interactions are predictable from the measured heats of surface adsorption and desorption of probe molecules from dilute solution. The heats of interaction are a measure of the strengths of these sites. Adhesion between basic aluminum foil and acidic polymer resin increases with increasing numbers of either acidic sites on the polymer or basic sites on the foil. The calorimetry and adhesion results are in good agreement. This study supports recent observations vide infra that wettability of the aluminum is much less important for polymer/aluminum adhesion than chemical bonding.

The contact angle (θ) of a sessile liquid drop on a horizontal solid surface can be calculated from the drop volume and the radius of the contact circle at the liquid/solid interface. A simple apparatus which allows simultaneous estimation of these two parameters is described, and tests of the method for two systems are reported. The first system is a 3.25 mole liter⁻¹ solution of 1-propanol in water on paraffin wax. The advancing (θ_a) and receding (θ_r) contact angles at 20°C are found to be (59.5 ± 1.0)° and (54.3 ± 0.3)°, respectively, in good agreement with the literature values and those found by direct measurement. The second system chosen is cyclohexane (a volatile liquid) on cleaved mica at 20°C. Two mica sheets were used. Mean contact angles of cyclohexane on the first mica sheet are θ_a = (7.45 ± 0.10)°, θ_r = (6.99 ± 0.12)°. For cyclohexane on the second sheet the mean contact angles are θ_a = (6.48 ± 0.31)°, θ_r = (5.56 ± 0.06)°. The difference between advancing and receding contact angles is statistically significant (P < 0.01) for both sheets. Other methods of comparable accuracy exist for θ ⪆ 30°, but the accuracy of most of these methods diminishes rapidly if θ ⪅ 30°. If calculation of the contact angle from the spacing between interference fringes is not appropriate, then estimation from drop volume and contact circle radius becomes the method of choice if θ <~ 30°.

Modification of polymer surfaces by changing the chemical structure, surface energy, and bonding characteristics has considerable technological importance in the area of adhesion. Reactive ion etching (RIE) and ion beam (IB) bombardment were employed to modify the surfaces of fully imidized 3,3',4,4'-biphenyl tetracarboxylic acid dianhydride-p-diaminophenyl (BPDA-PDA)-based polyimide (PI) films. These modification techniques affect only a shallow surface region, approximately 10-20 nm, and the bulk properties of the polymer are unaffected. The angle-resolved X-ray photoelectron spectroscopy (XPS) technique was used to characterize the PI surfaces modified by argon IB bombardment or oxygen RIE treatment. On the argon ion-bombarded surfaces, the XPS spectra indicate that the carbonyl and imide groups are decreased. Oxygen RIE treatment resulted in an increase in the atomic concentration of oxygen. To understand the surface aging effect, the freshly modified PI surfaces were exposed to laboratory air for 1 and 2 days. The changes in composition as a function of the depth of the modified surface region right after treatment and after aging were determined by the angle-resolved XPS technique (ARXPS). Contact angle measurements were used to determine the polar and dispersion components, the sum of which is the surface free energy. The polar component of the surface free energy shows the greatest change, with an increase of 8.0-9.4 times for both the oxygen RIE and ion beam treatments as compared with the as-cured PI surface. Aging of these modified surfaces resulted in a decrease of surface free energy as compared with the just-modified surfaces. In the case of oxygen RIE treatment, the dispersion component of the surface free energy showed little or no change from the as-cured sample. Adhesion of chromium/copper/chromium (Cr/Cu/Cr) films on PI was determined by peel strength measurements. Significant increases in peel strength, by a factor of 10-80, were shown for the modified surfaces. A good correlation between the peel strength and the experimentally determined polar component of surface energy was shown.

A remote plasma reactor, with air as the plasma gas, has been used for in-line surface modification of linear low-density polyethylene tape (LLDPE) passing 10 cm below the main plasma zone. Line speeds of up to 0.70 m/s were tested, allowing the study of 0.014 s exposure times to the plasma. Oxygen to carbon (O/C) ratios averaging 0.11 were observed on a reproducible basis. The reactor was also used for static plasma treatment under similar experiment onditions. This allowed a comparative study of short-term (milliseconds) vs. long-term (several seconds) plasma treatment. High-resolution X-ray photoelectron spectroscopy (XPS) analysis of the treated polymer surface suggested the formation of hydroxyl (COH), carbonyl (CO) and carboxyl (OCO) groups, even after short plasma treatment. The intensities of these components were seen to increase in approximately equal quantities with increasing O/C ratio. Water washing of polyethylene surfaces with high O/C ratios showed a loss of oxygen, apparent as a decrease in OCO groups in the C 1s spectra. A smaller loss in oxygen was observed when washing samples that had been plasma-treated for milliseconds. A surface ageing study revealed that polyethylene surfaces that had been plasma-treated for short time periods showed only a negligible loss of oxygen on prolonged exposure to air. Surfaces treated for longer time periods showed a loss of up to 50% of the total oxygen on the surface within a few days of treatment. Static secondary ion mass spectrometry has provided some supporting evidence for surface damage of the treated films.

The effects of remote nitrogen plasma, remote oxygen plasma, and corona discharge treatments on linear low-density polyethylene were studied with regard to the chemical and physical surface modification, depth of modification, and surface stability. An attempt was made to correlate the type and the extent of modification with the printing and adhesion properties of the modified surfaces. Surface topography was studied using scanning electron microscopy. The relative percentages of nitrogen and oxygen on the surfaces were determined by X-ray photoelectron spectroscopy. Printing and adhesion tests were performed using standard, commercially available inks and adhesives.

The effect of a remote nitrogen plasma on polyethylene and polystyrene was studied. The gas flow rate, the dilution of reactant gas, exposure times, and reactor base pressure were all found to have a large impact on the efficiency of nitrogen incorporation. Optimum conditions caused 18 atom % nitrogen to be incorporated within 20 seconds for polyethylene and 10 seconds for polystyrene. Studying a remote nitrogen plasma treated polyethylene sample over a period of 1 month indicated that except for a drop in the % N on initial exposure to air the concentration of nitrogen on the surface remained steady within the experimental limits. Angle resolved photoelectron spectroscopy indicated that nitrogen is incorporated to a depth below the analysis depth of XPS.

New reaction products have been generated on polyethylene and polystyrene surfaces using a novel two-step process. The first stage involves exposure to a downstream nitrogen plasma, and the second to either ozone or a corona discharge. It is observed that each of the two-step reactions yields very different reaction products, with an apparent increase in the formation of CO functional groups in the former case and the formation of surface NO₂ groups in the latter case.

The reaction rates and products of remote oxygen plasma treatment, corona discharge, and ozone treatment of high and low density polyethylenes have been examined using x-ray photoelectron spectroscopy. The oxygen uptake by remote plasma treatment was faster than that of other surface treatments using excited oxygen species. A steady state concentration of 18 ± 1% oxygen can be attained within 1 s of exposure in the remote plasma.

A study has been undertaken in which both x-ray photoelectron spectroscopy (XPS) and Fast atom bombardment static secondary ion mass spectrometry (FAB-SSIMS) have been used to study the effects of remote nitrogen plasma treatment on polymers such as linear low-density polyethylene (LLDPE), poly(ethylene vinyl alcohol) (EVOH) and poly(ethylene terephthalate) (PET). For comparison, remote oxygen plasma treatment was also performed on LLDPE. A very rapid uptake of nitrogen was observed for all polymers. Negative FAB-SSIMS indicated CN⁻, CNO⁻ and C₂N fragments on each of the nitrogen plasma-treated polmers. Positive FAB-SSIMS spectra of plasma-treated LLDPE showed relatively high intensity, high mass fragments, thought to originate from additives. These were not observed for the other two polymers. Significant amounts of aromatic-type fragments were observed in the positive FAB-SSIMS spectra of all treated polymers. Surface stability studies have shown that for both nitrogen and oxygen plasma-treaed LLDPE there is a substantial decrease in the surface functionality on exposure to air. This effect was much less prevalent for EVOH and PET.

Morphological and chemical changes of the surface of low-density polyethylene (LDPE), linear middle-density polyethylene (L-MDPE), and their 80/20 blend were studied by different techniques after corona-discharge treatment in air and subsequent annealing. The surface tension was determined by wetting; the roughness was measured by atomic force microscope (AFM), and the surface chemical composition was analyzed by X-ray photoelectron spectroscopy (XPS), whereas the low-molecular-mass fraction washed off by chloroform by FTIR. The surface tension of the films increases with the electrode current. The surface roughness depends primarily on the polymer type and is less affected by the corona treatment. At the initial stage of annealing, posttreatment-type oxidation and hydrophobic recovery are competing. The former is more pronounced in L-MDPE, the latter in LDPE. After annealing at 50°C for 160 days, hydrophobic recovery becomes predominant in each film studied, which is accompanied by significant smoothening of the surface. According to XPS and FTIR results, this is due to the migration of low-molecular-mass components (oligomers, oxidized polymer fractions, and additives) to the surface. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1529–1541, 2000
https://onlinelibrary.wiley.com/doi/abs/10.1002/%28SICI%291097-4628%2820000606%2976%3A10%3C1529%3A%3AAID-APP6%3E3.0.CO%3B2-J

One of the major disadvantages of low density polyethylene (LDPE) films is their poor adhesive properties. Therefore, LDPE films have been treated with atmospheric pressure air plasma in order to improve their surface properties. So as to simulate the possible conditions in an industrial process, the samples have been treated with two different sample distances (6 and 10 mm), and treatment rates between 100 and 1000 mm s⁻¹. The different sample distances are the distance of the sample from the plasma source. The variation of the surface properties and adhesion characteristics of the films were investigated for different aging times after plasma exposure (up to 21 days) using contact angle measurement, atomic force microscopy, weight loss measurements and shear test. Results show that the treatment increases the polar component and these changes improve adhesive properties of the material. After the twenty-first day, the ageing process causes a decrease of wettability and adhesive properties of the LDPE films (up to 60%).

The vast majority of polymers and composites have low surface energy; this is due to the low presence of functional groups on their surface which results in low adhesive properties. In order to modify this intrinsic property chemical and physical processes are commonly used. These processes present disadvantages, such as the use of products harmful to the environment. An alternative to these processes is the use of plasma technology. The main objective of this study is the improvement of the adhesive properties of the low density polyethylene (LDPE). In order to achieve the target, atmospheric plasma pretreatment has been optimized in order to promote subsequent adhesion processes, as the ones needed in the toy industry or the application of dyes or printing on surfaces. Plasma surface treatment is a clean process from the environmental viewpoint. This process does not emit any residue and it is easy to implement in an industrial process. Moreover the atmospheric plasma treatment is suitable to be applied in a large variety of materials even at high speeds when the treatment lasts less than a few seconds. In the present study it is examined the physical and chemical processes that occur in the LDPE surface as function of speed rate and distance of treatment. An increase both of the polar groups on the surface and the roughness after the treatment may increase its adhesive properties. It has been analyzed the influence of the speed rate and the nozzle distance on the final results. The adhesive properties have been evaluated using the T-peel test. The results show that at low speeds rates and low nozzle/substrate distance there is a greater inclusion of polar molecules at the surface. Consequently the adhesion properties of LDPE are improved.

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.