R&D Needs in the Drying of Coatings

The key technologies involved in the drying of coatings are first reviewed and then the factors that influence the drying process are identified, leading to a discussion uf future R&D needs. The drying mechanisms in the constant and falling rate periods are discussed. Present and nticipated estrictions on the use of solvents will result in aqueous systems replacing some solvent systems. and in wider use of high solids coatings and 100% solids coatings. Cross-linking reactions to solidify monomers and low-molecular polymers now become important. Dryers will become chemical reactors. Further development of models of drying, of diffusion, of surface solvent levels, and of solvent vapor pressures will be needed, along with those of dryer defects and drying stresses. To accurately simulate a wide range of coating systems more physical property data is needed. Nan-convection heating and environmental concerns complete the discussion.     

OPTIMIZATION OF SINGLE-ZONE DRYING OF POLYMER SOLUTION COATINGS TO AVOID BLISTER DEFECTS

ABSTRACT

The optimal conditions for drying polymer-solvent coatings result from a trade-off between minimizing the residual solvent level and creating defects. Blistering defects can be caused by boiling the solvent within the coating. In this paper, we use a detailed drying model with automated constrained optimization to find optimal drying conditions for prototypical coatings that minimize the residual solvent without blistering the coating. The drying oven is assumed to have a single zone with fixed residence time. The optimal drying conditions include the oven air temperature and substrate-side and coating-side heat transfer coefficients The latter are constrained to physically reasonable values. According to our results, the optimal coating-side heat transfer coefficient is always equal to or greater than the optimal substrate-side heat transfer coefficient.     

MICROENCAPSULATION BY SPRAY DRYING

Abstract

Spray drying technique has been widely used for drying heat-sensitive foods, pharmaceuticals, and other substances, because of the solvent rapid evaporation from the droplets, Although most often considered a dehydration process, spray drying can also be used as an encapsulation method when it entraps ‘active’ material within a protective matrix, which is essentially inert to the material being encapsulated. Compared to the other conventional microencapsulation techniques, it offers the attractive advantage of producing microcapsules in a relatively simple continuous processing operation. This chapter will present a brief overview of the main considerations involved in the application of spray drying for microencapsulation, with a special emphasis given to microencapsulation of volatile materials. The potential use of spray drying microencapsulation for pharmaceutical applications, particularly the preparation of microparticulate drug delivery systems, will also be discussed     

Measurements of Concentration Profiles in Polymeric Solvent Coatings by Means of an Inverse Confocal Micro Raman Spectrometer—Initial Results

Abstract

Concerning quality and drying time of a process the key issue is the progression of the concentration profiles within the film during drying. This contribution reports on the development of a new measuring technique, an Inverse-Confocal-Micro-Raman-Spectrometer, for online measurements of concentration profiles during drying in thin (5–200 µm) polymeric solvent coatings. The spatial and temporal resolution of the measurements is up to one micron and one second. A calibration method has been developed by taking Raman spectra of different, precisely concentrated samples in sealed quartz glass cells and by calculating the ratio of the intensities of the characteristic Raman peaks. The accuracy of this basic calibration method is very good over a wide concentration range. Initial investigations were carried out on the polymer solvent systems: polyvinyl acetate(PVAc)–toluene, PVAc–methanol, PVAc–benzene, and PVAc–methanol-toluene, PVAc–methanol-benzene.     

PORE EVOLUTION AND SOLVENT TRANSPORT DURING DRYING OF GELLED SOL-GEL COATINGS: PREDICTING ‘SPR1NGBACK’*

ABSTRACT

This paper reports predictions of drying phenomena in deformable porous gel coatings (i.e. a porous solid elastic network filled with air or solvent). Initially, a gelled coating is saturated with solvent, but as it dries, liquid-vapor menisci begin to recede into larger pores and the gel becomes a partially-saturated porous medium. The tensile capillary pressure in the liquid causes a compressive deformation on the solid skeleton and a consequent reduction in thickness and pore-size of the coating. A theory coupling the large deformation of the solid skeleton to capillary pressure in the interstitial liquid is used to predict the course of drying of dip-coated porous gel coatings. The theory predicts a ‘springback’ effect in late stages of drying as the effects of capillary pressure diminish, which matches with experimental observations.     

TRANSFER OPERATIONS IN PROCESS-INDUSTRIES: DESIGN AND EQUIPMENT

ABSTRACT

Drying of dielectric resin coatings is accompanied simultaneously by evaporation of multicomponent solvents and polycondensation from monomers. The characteristic of the drying is studied experimentally. As a test sample, a vanish consisting of trimellitic acid anhydride and 4.4¹-diphenylmethane diisocyanate dissolved in the mixture of N-methylpyrrolidone and xylene is coated on an aluminum pan. The sample is subjected in drying in two types of dryers: hot air heating and radiation heating. The constant drying rate period is not observed in any run. The maximum drying rate of the sample is lower than the evaporation rate from the solvent layer with no resin. There are remarkable fluctuations in the drying rate in the decreasing drying rate period. The fluctuations are caused by bubble formation. The progress of the reaction can be followed by IR spectroscopic analysis. From these results it is suggested that removal of the solvent and the product is inhibited by the formation of a polymer skin on the surface and it makes control of drying difficult.     

Carbon containing castables: current status and future prospects

Abstract

Measures to overcome the main technical di fficulties hindering the development and application of carbon containing castables are discussed. The aqueous wettability and dispersion properties of graphite can be improved by coating with materials such as carbides (SiC) and oxides (Al₂O₃, TiO₂, SiO₂, MgO, ZrO₂) or by forming micropellets / briquettes. Thick and dense crack free coatings are needed not only to improve the aqueous wettability and dispersion of graphite, but also its oxidation resistance. Small and dense micropellets or briquettes enable a homogeneous distribution of graphite in the matrix to be achieved, along with acceptable mechanical strength and corrosion resistance. Coating techniques have also been used to improve the hydration resistance of aluminium based antioxidants, but detailed studies in this area are still needed. The main binder systems are currently based on superfine silica fume and hydratable alumina as these do not form low melting phases in castables. As well as developing the existing coating and micropellet/briquette fabrication techniques, future work will continue to seek novel methods of incorporating graphite, and will begin to investigate installation methods and drying and heating schedules.     

THE USE OF AUXILIARIES IN DRYING TECHNOLOGY

ABSTRACT

In drying technology, the quality characteristics of the solid (its dispersibility, attrition resistance, etc.) are selectively adjusted by the use of auxiliaries. Typical auxiliaries include granulation aids for spray granulation and flow aids for solids with poor flow properties. Such auxiliaries are classified in the first part of this paper according to their mechanisms of action (eg. their adhesive effect, dispersion effect, bursting effect).

In the second part of the paper, an example will be used to illustrate the effect of auxiliaries. The effect of a binder (salt or polymer) on the drying of free-floating suspended droplets is investigated. The fracture resistance of the particles and the required drying time increase as the proportion of dissolved components increases.     

Deep-Bed Grain Drying - A Review of Particular Theories

ABSTRACT

Theories developed for explaining the drying phenomena in grain drying systems are referred to as particular grain drying theories. The objectives of this paper are to review various particular grain drying theories and to suggest directions for future research in this area.

Grain drying models are classified as non-equilibrium, equilibrium, and logarithmic type. Four non-equilibrium models (stationary bed, cross-, concurrent-, and counter-flow) were reviewed. Under low temperature and low airflow conditions, drying in a stationary deep-bed occurs at near equilibrium. The derivation of an equilibrium grain drying model and its necessary assumptions were discussed. Logarithmic models were developed and applied by many researches mostly to low temperature drying in stationary deep-beds. A semi-empirical thin-layer equation is commonly used in the reported grain drying models. The inclusion of this equation is an important factor, that determines the cognitive and practical values of a grain drying model. Due to this equation, the discussed models lack the capability of explaining the drying process by the laws of diffusion and the mathematical theory of drying solids. The computer simulation of grain drying systems can provide information for design and improvement of grain drying systems.     

Microwave-Vacuum Drying Kinetics of Pharmaceutical Powders

Abstract

The drying kinetics (maximum drying rate, drying constants) and center temperature of selected powder (aspirin, paracetamol, lactose, and maize starch)–solvent (water, ethanol, methanol, and acetone) systems were monitored during microwave-vacuum drying. An experimental microwave-vacuum system (650 W and 2.45 GHz) operated at 61–81 kPa was used. The drying rate profile did not vary with the powder–solvent system; an initial warming-up period was followed by a constant-rate stage and two falling-rate periods. However, the drying kinetics were found to be both powder and solvent dependent, with the drying times for acetone-, ethanol-, and methanol-wetted materials being considerably shorter (up to 89.8%) than those of samples containing water. Although the general form of the temperature profile (short warming-up period, constant-rate stage, and decreasing temperature phase) was similar for all systems, the maximum temperatures varied quite significantly with solvent type, ranging from highest to lowest in the order water-ethanol-methanol-acetone. For most powder–solvent systems, reduced operating pressure facilitated increased drying rates and thus shorter drying times.     

Beneficiation of Pu Residues by Ultrafine Grinding and Aqueous Biphasic Extraction

Abstract

Aqueous biphase systems are heterogeneous liquid/liquid systems that result from appropriate combinations of inorganic salts and water-soluble polymers such as polyethylene glycol. Colloid-size particles that are suspended in an aqueous biphase system will partition to one of the phases, depending on a complex balancing of particle interactions with the surrounding solvent. With regard to waste treatment applications, aqueous biphase systems are similar to conventional solvent extraction but do not utilize an organic diluent, which may itself become a source of pollution. In addition, the water-soluble polymers that have been used in biphase formation are inexpensive, nontoxic, and biodegradable. The application of aqueous biphasic extraction to the beneficiation of plutonium residues will be discussed.     

TWO-SIDED DRYING OF COATINGS ON POROUS WEBS

ABSTRACT

A relatively simple spreadsheet-based model of the drying of coated webs has been extended to cover coatings on porous webs Two cases have been modeled. In one the coating is assumed to fully saturate the porous web, with the drying being completely symmetrical about the center plane except for differences in the air velocities and temperatures. The drying rate in the constant rate period is then increased, but because of the greater thickness that the coating now occupies, diffusion is much slower and the falling rate drying can be slower, even though it takes place on both sides. In the other case the coating is assumed to lie on top of the porous web, with solvent vapor diffusing through the web. Here the constant rate drying is only slightly faster than in the base case, but in the falling rate period the drying can be greatly accelerated Both models predict the temperatures and solvent levels throughout the length of the dryer.     

Organic solvents in high solids and water-reducible coatings

One answer to reduced levels of solvent in coatings is higher solids. Another is water-reducible coatings. These still contain volatile ingredients in addition to water, including various organic solvents and amines. This report describes recent research on the behavior of volatile organic materials in high solids and water-reducible coatings in terms of the technical benefit/need point of view. It is hoped that better understanding will lead to their more judicious use.

Topics discussed include: solvency, surface tension control, viscosity control, flashpoints/fire hazard, evaporation phenomena, solvent retention, and amine evaporation. Environmental factors are not specifically included.     

Reduction of zinc and volatile organic solvents in two-pack anti- corrosive primers, a pilot study

For decades one of the directions to obtain anti-corrosive properties of coatings in the protective coating industry has been the use of zinc as an active pigment in the primer layer. However due to strengthening of environmental legislation the use of this metal has become an item of discussion. In addition due to the nature of the traditional low solid zinc containing formulations, formulated close to critical pigment volume concentration (PVC), it proved to be difficult to develop very high solids zinc containing formulations with comparable performance. At the moment the product range of our company contains both zinc containing and zinc free two-pack primers. Examples from both generic types have excellent track records. To determine the position of the zinc free primers in relation to the zinc containing ones a comparison study was initiated. A new high solids two-pack epoxy primer was included in this test program to check whether a solvent reduction in this range of products was feasible. In this paper the results of the comparison between a zinc epoxy primer, a zinc free epoxy primer and a high solids version of the zinc free epoxy primer, some formulation aspects and the consequences for the future will be discussed.     

CALCULATION OF VACUUM DRYING OF DISPERSED MATERIALS FROM MULTICOMPONENT LIQUID SYSTEMS

ABSTRACT

Calculation methods for different types of drying operations (vacuum-coductive drying, drying by pressure drop, self-freezing before sublimation) from multiliquid systems were presented. Kinetic relations of vacuum drying were obtained which allow calculation of drying equipment and choise of optimal operating conditions. The existence of balance invariants removing multicomponent liquid systems - single-valued relations between pressure, moisture content, liquid and exhausted vapour phase composition and product temperature were found. These relations are used to reduce the number of experiments needed to analyze the dryer.     

THE USE OF A GAS CHROMATOGRAPH TO OBTAIN DRYING RATES DATA THROUGH SIMPLE LABORATORY EXPERIMENTS

Abstract

A method making use of a gas chromatograph was investigated to acquire bound solvent drying rates. The solid to be dried was used as stationary phase in a chromatographic column, and the TCD detector was used to follow the decrease of solvent content of the effluent as inert gas was flown through the column. The measurement of the break-through curve allows the evolution of drying rate with time or solvent content to be obtained. The comparison of the experimental results with numerical simulation leads to the determination, through parametric adjustment, of the internal mass-transfer coefficient as a function of the solvent content during the second stage of bound solvent drying period. This parameter can lead to the value of the diffusivity as a function of solvent content.     

Effect of the hydroxyl content and molecular weight of polyvinyl butyral on tape properties

Despite the desire to replace organics with water in tape casting systems, thick films often use solvent in order to speed up drying rates. Polyvinyl butyral having hydroxyl contents ranging between 11 and 20% and molecular weights between 40,000 and 120,000 g/mol was plasticized with butyl benzyl phthalate at solids contents between 62 and 75 vol.% La_0.5Sr_0.5CoO₃ once the toluene/ethanol solvent was evaporated. High-hydroxyl content was more important than high molecular weight in increasing the Young's modulus, yield strength, and tensile strength of the tapes at room temperature. Annealing tapes at low temperatures (65 °C) for short times (10 min) relaxed stresses and tended to equalize modulus, strength, and ductility. Low hydroxyl content and low molecular weight enhanced particle packing, which resulted in cracking as tape thickness increased from 0.2 mm to 0.7 mm at higher solids contents. Crack-free tapes were made at all solids contents when the hydroxyl content was greater than 18%. Lower molecular weight at high-hydroxyl content, along with high solids content, was important for minimizing the amount of solvent used.     

Design of Binary Polymeric Coatings for Minimizing the Residual Solvent,Part I: Experimentation

Several binary polymeric coatings of poly(styrene)–tetrahydrofuran have been dried under quiescent drying conditions. Coatings are made using four different solutions of polymer having 5.01, 9.75, 10.06, and 15.14 wt% poly(styrene) initially. Experiments were performed for several different thicknesses for each polymer solution. Residual solvent remaining was lowest in the thicker coating. Results show that the thicker coating should be applied once rather than layer by layer to minimize the remaining residual solvent. The next layer is applied after complete drying of the previous layer. This practice should continue until the desired thickness is obtained. For instance, coatings of initial thickness 1010 µ m and final thickness 61 µm have residual solvent which is lower than the residual solvent in dried coatings of nearly the same final thicknesses using the layer-by-layer technique.     

Residual solvent minimization in polystyrene-p-xylene coatings using a nonvolatile additive

The use of plasticizer has been extensively studied for enhancing the flexibility, processability, solubility, and vapor permeability of polymeric materials. However, its use in enhancing drying of polymeric coatings has been scarcely studied. In the present work, the optimization of plasticizer loading for drying characteristics and residual solvent minimization in polystyrene-p-xylene polymeric coatings has been investigated. Coatings were prepared by solution casting technique at various triphenyl phosphate (TPP) loadings from 0 to 7.50 wt% for two cases of initial solution volumes of 75 and 125 μl. For these two cases, respectively, the minimum residual solvent values of 0.02% and 1.61% were obtained, both at 2.50 wt% TPP loading. However, the two cases exhibited residual solvent values of 2.32% and 4.45% at 0 wt% TPP. Further, the SEM results also showed that at 2.50 wt% TPP loading, the coatings developed a dense, smooth and defect-free morphology attributed to their minimized residual solvent content. The coatings, however, had a porous and phase-separated morphology originally at 0 wt% TPP. The variation of residual solvent, coating thickness and drying rate with plasticizer loading followed the same behavior for different initial solution volumes. All these parameters attained their peak values consistently at 2.50 wt% TPP. The study is first of its kind to indicate massive reduction in residual solvent attributed to the addition of a nonvolatile plasticizer. TPP also imparts flame retardance to inherently inflammable polystyrene thereby facilitating its implementation in electronic devices. The obtained coatings have possible applications in electronics, electro-optics, and food packaging.