A basic understanding of rheology and surface chemistry, two primary sciences of liquid flow and solid-liquid interaction is necessary for understanding coating and printing processes and materials. A generally qualitative treatment of these subjects will suffice to provide the insight needed to use and apply coatings and inks and to help solve the problems associated with their use. Rheology, in the broadest sense, is the study of the physical behavior of all materials when placed under stress. Four general categories are recognized: elasticity, plasticity, rigidity, and viscosity. Our concern here is with liquids and pastes. The scope of rheology of fluids encompasses the changes in the shape of a liquid as physical force is applied and removed. Viscosity is a key rheological property of coatings and inks. Viscosity is simply the resistance of the ink to flow-the ratio of shear stress to shear rate. Throughout coating and printing processes, mechanical forces of various types and quantities are exerted. The amount of shear force directly affects the viscosity value for non-Newtonian fluids. Most coatings undergo some degree of" shear thinning" phenomenon when worked by mixing or running on a coater. Heavy inks are especially prone to shear thinning. As shear rate is increased, the viscosity drops, in some cases, dramatically. This seems simple enough except for two other effects. One is called the yield point. This is the shear rate required to cause flow. Ketchup often refuses to flow until a little extra shear force is applied. Then it often flows too freely. Once the yield point has been exceeded the solidlike behavior vanishes. The loose network structure is broken up. Inks also display this yield point property, but to a lesser degree. Yield point is one of the most important ink properties.

A review of studies of polymer-paper adhesion illustrates the thermodynamic nature of the bondability of polymers to plain, uncoated paper surfaces. The bond strength depends strongly on the chemical nature of the polymer surface and on that of the fibrous paper surface. Adhesion to paper may be characterized indirectly through thermodynamic analysis of the paper substrate, or directly through paper laminate or adhesion tape peel testing. The need for adequate paper adhesion is emphasized, particularly for some of the newer printing processes (electrophotographic and thermal imaging). It is concluded that some of the indirect methods of adhesion characterization (surface energetics analysis via contact angle measurements or the inverse gas chromatography technique) may serve to characterize paper adhesion in these processes.

An oriented, heat sealable polypropylene film is provided having a metallizable surface. The film includes a core layer derived from isotactic polypropylene containing an effective amount of adhesion promoting agent. A copolyester layer is bonded to the core layer, the adhesion promoting agent protecting against the delamination thereof. A heat sealable layer formed from an ethylene-propylene random copolymer is bonded to the opposite side of the core layer. The film is formed as a coextrudate and is biaxially oriented.

X-ray photoelectron spectroscopy (ESCA), wettability measurements, and an ink adhesion test were used to characterize changes in the surface properties of air-corona-treated polypropylene (PP) films upon aging under a variety of storage conditions. No changes in ESCA O/C atomic ratios as a function of aging were observed for corona-treated PP films. The wettability data indicated a slight decrease in wettability upon aging. Aging did not affect ink adhesion for the particular PP and ink studied. The responses obtained were independent of the various film storage conditions employed. The slight decrease in wettability observed upon aging was attributed to reorientation of oxidized functionalities within the surface region.

The effects of ion irradiation on polyimide surfaces have been studied using x‐ray photoemission techniques. Ion bombardment with energies in the range 0.5–2.0 keV and doses between 8×10¹³ and 1×10¹⁵ ions/cm² were carried out in situ in the x‐ray photoelectron spectrometer and the chemistry of the modified surface was monitored using core level spectral changes. At low doses and energies, carbonyl groups were preferentially sputtered keeping the rest of the monomer intact. Loss of nitrogen was insignificant compared to losses of carbon and oxygen. At higher energies and doses, the polymer undergoes extensive bond scission, restructuring of various functional groups and species, together with radical and anion formation. High resolution spectra indicated a binding energy scale shift to a lower value, which increased with ion energy and dose, and which was related to the creation of a surface negative charge. The effects of exposure to moisture in the ambient on the surface charge, on the surface structure, and on the surface chemistry was studied.

The surface properties of three undecyl oxazoline homopolymers and two phenyl/undecyl oxazoline block copolymers (as comparison) were studied. After coating on glass slides and annealing, all films had a low critical surface energy of 21 dynes/cm. Water contact angles were higher than 107° for the most hydrophobic films. The deduction that the polymer surfaces contained close-packed methyl groups was further confirmed by electron spectroscopy chemical analysis (ESCA) angle profiling on an annealed undecyl oxazoline homopolymer film. A model was developed for the variation of elemental ratios as a function of photoelectron take-off angle. This verified that the polymer films had the polymer backbones parallel to the surface with the undecyl tails oriented toward the surface. When these block and homopolymers were coated on copy paper and glass slides, the peel strengths of pressure-sensitive adhesives with these surfaces were very low for short dwell times at room temperature. At long dwell times or at elevated temperatures, the peel strengths remained low for the homopolymers but increased greatly for the block copolymers to values higher than those in the tape on glass. After 24 h at 70°C, ESCA analysis showed that the adhesive diffused into the phenyl block domains of the diblock copolymer, generating high peel strength and cohesive failure. However, under the same annealing conditions, the triblock copolymer showed adhesive failure while peel strength increased. ESCA analysis showed very litle diffusion of the adhesive into the triblock copolymer. The homopolymers were stable toward vinyl acetate type adhesives even at elevated temperature; they were abhesive up to 100°C with no interdiffusion.

Recent work in our laboratories has fully characterized the surface region of a segmented poly(ether-urethane) (PEU) extending from the air/polymer interfacial region through bulk depths in the micron range. This characterization utilized energy and angle dependent Electron Spectroscopy for Chemical Analysis (ESCA), Attenuated Total Reflectance–Fourier Transform Infrared Spectroscopy (ATR–FTIR), and Comprehensive Wettability Profiling (contact angle using a homologous series of liquids) as defined by Zisman. In this study this same multi-analytical-technique approach is used to elucidate changes in these PEU surfaces induced through an H₂O Radio Frequency Glow Discharge (RFGD) plasma. This investigation reports both qualitative and quantitative changes due to the modification treatments as well as the permanency of the changes effected on these surfaces through the plasma treatment. From our analyses, the amount of surface residing polyurethane (hard segment) is observed to increase due to a proposed plasma etching mechanism. Further, the addition of oxygen containing functionality is detected at the modified surfaces unique with respect to the unmodified PEU. These surface modifications which show large increases in wettability, are finally observed to be semi-permanent over a time period of 6 months.

The contact angle of water on several polymer films was determined by three different methods; telescopic sessile drop, laser beam goniometry, and the Wilhelmy plate technique. The telescopic sessile drop method is the simplest, but the least accurate; whereas the laser beam goniometry compares favorably with the Wilhelmy plate in terms of accuracy, but cannot easily provide information on contact angle hysteresis.

How to convert an analytical balance into an accurate ring tensiometer or densimeter.

It was found that the treatment of the surfaces of wet pulp sheets (moisture content; up to 85%) in a corona discharge improved greatly the plybond strength of the paperboard obtained when the treated wet pulp sheets were laminated together, pressed, and then dried. Treatment was carried out by use of a corona apparatus which had variable driven roll electrodes for transporting the wet pulp sheets through a corona field and was attached to a high-voltage generator (∼ max 500 W, ∼ 16 kV at 5 kHz). The plybond strengths of the paperboards were examined by means of Tappi RC-273 and JIS P8139 methods. Some experiments regarding the chemical effects of the corona treatment on the surface modification of wet pulp sheets were made with the aid of dye adsorption methods. Both untreated and corona-treated pulps adsorbed basic dyes, methylene blue, etc., with the same extent of dyeing. This indicates that no measurable acidic sites (carboxyl groups) increased on the surfaces of the pulp sheets during the corona treatment. To detect aldehyde groups, the dyeing examination of the pulps with Schiff's reagent was made, and the results showed a higher dyeing ability for the corona-treated pulps compared to the untreated, indicating that aldehyde groups on the pulp surfaces increased with an increase in the degree of corona treatment. The corona treatment seems to produce on the surface layer lightly oxidized and fairly degraded polysaccharide chains, which will tend to swell in water and thus act as an adhesive in plybonding the pulp sheets.

Polystyrene tissue culture vessels are commercially treated by corona discharge or plasma surface oxidation to provide a hydrophilic surface, with 15–20% surface oxygen. ESCA and FTIR showed that oxidation forms hydroxyl, carbonyl, and carboxyl groups. We have discovered that water washing removes about half the oxidized species. It is believed that reaction with the vinyl polymer backbone to form carboxyl groups results in CC bond scission to form soluble fragments; addition and ring reactions would not yield soluble species. This functional group removal could affect the desired properties, such as the use of these groups as anchors in chemical coupling.

SSIMS (static secondary ion mass spectroscopy) has been used to aid in the interpretation of hydrophobic recovery of oxygen plasma treated PS (polystyrene), together with XPS (X-ray photoelectron spectroscopy) and water contact angle measurements.

The heterogeneity in sputter yields of different ions did not allow complete qualitative and quantititative information to be obtained. Yet negative ion spectra of surfaces treated with plasmas of isotopically enriched oxygen allowed us to follow closely the disappearance of polar groups during hydrophobic recovery. Furthermore, using isotopically enriched PS samples, it was possible to obtain unambiguous information about molecular weight and temperature-induced changes in the hydrophobic recovery mechanism, which could not have been provided by XPS.

XPS, SEM, SSIMS, FTIR-ATR, water-in-air, and air-in-water contact angle measurements have been used to unambiguously characterize the locus of failure of PP/epoxy joints. In the case of untreated PP, the fracture has been found adhesive, whereas in oxygen plasmatreated PP, it is cohesive, within bulk PP, but close to the modified PP-bulk PP interface. The smoothness of fracture surfaces allowed us to exclude mechanical interlocking effects. Shear-strength measurements showed that the mechanical strength of the joint was improved by plasma treatment. Preliminary thermal equilibration of the plasma-treated PP sample and changes in the curing cycle of the epoxy resin did not change either the locus of failure or the shear strength of the joint. The reason is probably because the number of polar functions left at the surface after thermal equilibration is sufficient to induce adhesion. The mechanical strength of the PP surface layer may be the determining factor. Fracture energy calculations showed that the observed locus of failure is the same as predicted on the basis of surface energy considerations.

Oxygen plasma treatment of polypropylene (PP) surfaces led to introduction of oxygencontaining functionalities, with consequent improvement of surface wettability. A combination of X-ray photoelectron spectroscopy (XPS), static secondary ion mass spectroscopy (SSIMS), and contact angle measurements (water-in-air and air-in-water) allowed us to characterize the behavior of the treated surface in contact with air (low-energy surface) and water (high-energy surface).

The treated surface showed the tendency to rearrange itself to minimize its interfacial energy. When contacted with air (low-energy surface), polar groups were buried away from the polymer/air interface, while in contact with water (high-energy surface) polar groups remained at the polymer/water interface.

When contacted with air, the polymer surface layer rearranged by macromolecular motions within itself, since interdiffusion with the bulk polymer seems forbidden. These motions are thermally activated and it was possible to obtain an apparent activation energy (58.1 kJ/mol) close to those reported for other vinyl polymers.

Surface structures of poly(ethylene terephthalate) films produced by stretching after Ar plasma-treatment were analyzed by X-ray photoelectron spectroscopy, combined with a gas chemical modification technique, secondary ion mass spectrometry, and transmission electron microscopy. The formation of ultra-fine protrusions that produce good slip and a smooth film surface after the stretching procedure was confirmed. The height of the protrusions was almost the same as the thickness of the osmic acid-dyeable layer. The thickness of the modified layer was found to change according to the Ar pressure of the plasmatreatment. Carboxylic and hydroxyl groups produced by the plasma-treatment were quantified. The formation of these functional groups can explain good adhesion of the film.

Characterization of poly(ethylene terephthalate) (PET) films surfaces through wettability measurements and inverse gas chromatography techniques leads to a better knowledge of the potential interactions with a coating. An important case is the one relative to gelatin coatings for photographic films. In order to favor adhesion on PET, it is of interest to examine the problem in terms of acid–base interactions. PET is found amphoteric and gelatin rather basic. Several surface treatments on PET like orientation on water and flame or plasma treatment in air lead to an increase in surface acidity. Adhesion with gelatin as determined by the peel test is increased through a flame treatment, because of the higher acidity of PET and subsequent chemical bonding at the interface. Determination of acid-base surface properties of PET and gelatin appears to be an excellent tool for adhesion prediction.

The surface of polyester grafted with acrylic acid has been characterized using contact angle measurements of a two-liquid phase system and FT-IR and ESCA spectroscopy as a function of the concentration of acrylic acid on grafting. The COOH groups on the polymer surface influence only the polar component γ_s ^p of surface energy and not the dispersive one γ_s ^d. Both the FT-IR and ESCA characterizations, showing the transformation of COOH to COONa by alkaline treatment, provide information with a high degree of surface sensitivity, comparable to that of contact angle measurements. The relative area ratios of the COONa peak to the COOR peak by FT-IR ( A_surface) and of the Na_1s peak to the C_1s peak by ESCA are linearly correlated to γ_s^p.

The use of plasma deposition to introduce sulfonate groups to the surface of a polyurethane was attempted. In previous work, the bulk incorporation of sulfonate groups was found to improve the blood contacting properties of the base polyurethane but physical properties in the hydrated state were adversely affected. Plasma deposition schemes involving ammonia and sulfur dioxide were utilized in an attempt to incorporate sulfonate groups. Surface characterization by X-ray photoelectron spectroscopy (XPS) and contact angle measurements was used to follow polymer surface rearrangement dynamics and to address the issue of plasma chemistry specificity. Concerns of reaction specificity were alleviated by using the plasma as a pretreatment which is followed by a chemical surface derivatization.

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.

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.

Oxygen plasma treatment can be used for increasing the hydrophilicity of polymer surfaces, however, it is widely known that this effect decays significantly with time. This ageing phenomenon is thought to be caused by both migration of low molecular weight fragments and reorientation of modified polymer chains. It has recently been shown that the aged surface becomes transiently hydrophilic before attaining a final surface energy significantly lower than the initially treated surface. X-ray photoelectron spectroscopy (XPS) and contact angle measurements were used to monitor the changes in surface chemistry of plasma oxidized poly(ether ether ketone) (PEEK) during post treatment storage. The decay and transient increase in hydrophilicity were found to be dependent upon crystallinity and storage temperature.