Powder coating is a type of coating, applied as a free-flowing, dry powder. The main difference between a conventional liquid paint and a powder coating is that the powder coating does not require a solvent to keep the binder and filler parts in coating and is cured under heat to allow it to flow and form a "skin"; the powder may be a thermoset polymer. It is used to create a hard finish, tougher than conventional paint. Powder coating is used for coating of metals, such as household appliances, aluminium extrusions, drum hardware and automobile and bicycle parts. Newer technologies allow other materials, such as MDF, to be powder coated using different methods; the powder coating process was invented around 1945 by Daniel Gustin US Patent 2538562. Because powder coating does not have a liquid carrier, it can produce thicker coatings than conventional liquid coatings without running or sagging, powder coating produces minimal appearance differences between horizontally coated surfaces and vertically coated surfaces.
Because no carrier fluid evaporates away, the coating process emits few volatile organic compounds. Several powder colors can be applied before curing them all together, allowing color blending and bleed special effects in a single layer. While it is easy to apply thick coatings that cure to smooth, texture-free coating, it is not as easy to apply smooth thin films; as the film thickness is reduced, the film becomes more and more orange peeled in texture due to the particle size and glass transition temperature of the powder. Most powder coatings have a particle size in the range of 2 to 50 μ, a softening temperature Tg around 80 °C, a melting temperature around 150 °C, are cured at around 200 °C. for minimum 10 minutes to 15 minutes. For such powder coatings, film build-ups of greater than 50 μ may be required to obtain an acceptably smooth film; the surface texture, considered desirable or acceptable depends on the end product. Many manufacturers prefer to have a certain degree of orange peel since it helps to hide metal defects that have occurred during manufacture, the resulting coating is less prone to showing fingerprints.
There are specialized operations where powder coatings of less than 30 micrometres or with a Tg below 40 °C are used in order to produce smooth thin films. One variation of the dry powder coating process, the Powder Slurry process, combines the advantages of powder coatings and liquid coatings by dispersing fine powders of 1–5 micrometre particle size into water, which allows smooth, low film thickness coatings to be produced. For garage-scale jobs, small "rattle can" spray paint is less expensive and complex than powder coating. At the professional scale, the capital expense and time required for a powder coat gun and oven are similar to a spray gun system. Powder coatings have a major advantage. However, if multiple colors are being sprayed in a single spray booth, this may limit the ability to recycle the overspray. Powder coatings contain no solvents and release little or no amount of Volatile Organic Compounds into the atmosphere. Thus, there is no need for finishers to buy costly pollution control equipment.
Companies can comply more and economically with the regulations of the U. S. Environmental Protection Agency. Powder coatings can produce much thicker coatings than conventional liquid coatings without running or sagging. Powder coated items have fewer appearance differences than liquid coated items between horizontally coated surfaces and vertically coated surfaces. A wide range of speciality effects are accomplished using powder coatings that would be impossible to achieve with other coating processes. Curing time is faster with powder coating than with liquid coating. There are two main categories of powder coating: thermoplastics; the thermosetting variety incorporates a cross-linker into the formulation. When the powder is baked, it reacts with other chemical groups in the powder to polymerize, improving the performance properties; the thermoplastic variety does not undergo any additional actions during the baking process as it flows to form the final coating. The most common polymers used are: polyester, polyester-epoxy, straight epoxy and acrylics.
The polymer granules are mixed with hardener and other powder ingredients in an industrial mixer, such as a turbomixer The mixture is heated in an extruder The extruded mixture is rolled flat and broken into small chips The chips are milled and sieved to make a fine powder The powder coating process involves three basic steps: Part preparation or the pre-treatment The powder application Curing Removal of oil, lubrication greases, metal oxides, welding scale etc. is essential prior to the powder coating process. It can be done by a variety of mechanical methods; the selection of the method depends on the size and the material of the part to be powder coated, the type of impurities to be removed and the performance requirement of the finished product. Chemical pre-treatments involve the use of phosphates or chromates in submersion or spray application; these occur in multiple stages and consist of degreasing, etching, de-smutting, various rinses and the final phosphating or chromating of the substrate & new nanotechnology chemical bonding.
The pre-treatment process both improves bonding of the powder to the metal. Recent additional processes have been developed that avoid the use of chromates, as these can be toxic to the
Dichloromethane is a geminal organic compound with the formula CH2Cl2. This colorless, volatile liquid with a moderately sweet aroma is used as a solvent. Although it is not miscible with water, it is polar, miscible with many organic solvents. Natural sources of dichloromethane include oceanic sources, macroalgae and volcanoes. However, the majority of dichloromethane in the environment is the result of industrial emissions. DCM is produced by treating either chloromethane or methane with chlorine gas at 400–500 °C. At these temperatures, both methane and chloromethane undergo a series of reactions producing progressively more chlorinated products. In this way, an estimated 400,000 tons were produced in the US, Japan in 1993. CH4 + Cl2 → CH3Cl + HCl CH3Cl + Cl2 → CH2Cl2 + HCl CH2Cl2 + Cl2 → CHCl3 + HCl CHCl3 + Cl2 → CCl4 + HClThe output of these processes is a mixture of chloromethane, dichloromethane and carbon tetrachloride; these compounds are separated by distillation. DCM was first prepared in 1839 by the French chemist Henri Victor Regnault, who isolated it from a mixture of chloromethane and chlorine, exposed to sunlight.
DCM's volatility and ability to dissolve a wide range of organic compounds makes it a useful solvent for many chemical processes. Carbon diselenide is produced by reacting selenium powder with dichloromethane vapor near 550°C. 2 Se + CH2Cl2 → CSe2 + 2 HClIt is used as a paint stripper and a degreaser. In the food industry, it has been used to decaffeinate coffee and tea as well as to prepare extracts of hops and other flavorings, its volatility has led to its use as an aerosol spray propellant and as a blowing agent for polyurethane foams. The chemical compound's low boiling point allows the chemical to function in a heat engine that can extract mechanical energy from small temperature differences. An example of a DCM heat engine is the drinking bird; the toy works at room temperature. DCM chemically welds certain plastics. For example, it is used to seal the casing of electric meters. Sold as a main component of plastic welding adhesives, it is used extensively by model building hobbyists for joining plastic components together.
It is referred to as "Di-clo." It is used in the garment printing industry for removal of heat-sealed garment transfers, its volatility is exploited in novelty items: bubble lights and jukebox displays. DCM is used in the material testing field of civil engineering. DCM is the least toxic of the simple chlorohydrocarbons, but it is not without health risks, as its high volatility makes it an acute inhalation hazard, it can be absorbed through the skin. Symptoms of acute overexposure to dichloromethane via inhalation include difficulty concentrating, fatigue, headaches, numbness and irritation of the upper respiratory tract and eyes. More severe consequences can include suffocation, loss of consciousness and death. DCM is metabolized by the body to carbon monoxide leading to carbon monoxide poisoning. Acute exposure by inhalation has resulted in optic hepatitis. Prolonged skin contact can result in DCM dissolving some of the fatty tissues in skin, resulting in skin irritation or chemical burns, it may be carcinogenic, as it has been linked to cancer of the lungs and pancreas in laboratory animals.
Other animal studies showed salivary gland cancer. Research is not yet clear as to. DCM crosses the placenta. Fetal toxicity in women who are exposed to it during pregnancy, has not been proven. In animal experiments, it was fetotoxic at doses that were maternally toxic but no teratogenic effects were seen. In people with pre-existing heart problems, exposure to DCM can cause abnormal heart rhythms and/or heart attacks, sometimes without any other symptoms of overexposure. People with existing liver, nervous system, or skin problems may worsen after exposure to methylene chloride. In many countries, products containing DCM must carry labels warning of its health risks. In February 2013, the U. S. Occupational Safety and Health Administration and the National Institute for Occupational Safety and Health warned that at least 14 bathtub refinishers have died since 2000 from DCM exposure; these workers had been working alone, in poorly ventilated bathrooms, with inadequate or no respiratory protection, no training about the hazards of DCM.
OSHA has since issued a DCM standard. In the European Union, the European Parliament voted in 2009 to ban the use of DCM in paint-strippers for consumers and many professionals; the ban took effect in December 2010. In Europe, the Scientific Committee on Occupational Exposure Limit Values recommends for DCM an occupational exposure limit of 100 ppm and a short-term exposure limit of 200 ppm. Concerns about its health effects have led to a search for alternatives in many of these applications. On March 15, 2019, the U. S. Environmental Protection Agency issued a final rule to prohibit the manufacture and distribution of methylene chloride in all paint removers for consumer use, effective in 180 days. Dichloromethane is not classified as an ozone-depleting substance by the Montreal Protocol; the U. S. Clean Air Act does not regulate dichloromethane as an ozone depleter. According to the EPA, the atmospheric lifetime of dichloromethane is short, such that the substance decomposes before reaching the ozone layer.
Ozone concentrations measured at the midlatitudes from the g
A mesh strainer known as sift known as sieve, is a device for separating wanted elements from unwanted material or for characterizing the particle size distribution of a sample using a woven screen such as a mesh or net or metal. The word "sift" derives from "sieve". In cooking, a sifter is used to separate and break up clumps in dry ingredients such as flour, as well as to aerate and combine them. A strainer is a form of sieve used to separate solids from liquid; some industrial strainers available are simplex basket strainers, duplex basket strainers, Y strainers. Simple basket strainers are used to protect valuable or sensitive equipment in systems that are meant to be shut down temporarily; some used strainers are bell mouth strainers, foot valve strainers, basket strainers. Most processing industries will opt for a self-cleaning strainer instead of a basket strainer or a simplex strainer due to limitations of simple filtration systems; the self-cleaning strainers or filters are more efficient and provide an automatic filtration solution.
Sieving is a simple technique for separating particles of different sizes. A sieve such as used for sifting flour has small holes. Coarse particles are separated or broken up by grinding against screen openings. Depending upon the types of particles to be separated, sieves with different types of holes are used. Sieves are used to separate stones from sand. Sieving plays an important role in food industries where sieves are used to prevent the contamination of the product by foreign bodies; the design of the industrial sieve is here of primary importance. Triage sieving refers to grouping people according to their severity of injury; the mesh in a wooden sieve might be made from wicker. Use of wood to avoid contamination is important. Henry Stephens, in his Book of the Farm, advised that the withes of a wooden riddle or sieve be made from fir or willow with American elm being best; the rims would be made of fir, oak or beech. A sieve analysis is a practice or procedure used to assess the particle size distribution of a granular material.
Sieve sizes used in combinations of four to eight sieves. Designations and Nominal Sieve Openings Chinois, or conical sieve used as a strainer sometimes used like a food mill Cocktail strainer, a bar accessory Colander, a bowl-shaped sieve used as a strainer in cooking Flour sifter or bolter, used in flour production and baking Graduated sieves, used to separate varying small sizes of material soil, rock or minerals Mesh strainer, or just "strainer" consisting of a fine metal mesh screen on a metal frame Riddle, used for soil Spider, used in Chinese cooking Tamis known as a drum sieve Tea strainer intended for use when making tea Zaru, or bamboo sieve, used in Japanese cooking Cheesecloth Cloth filter Gold panning Gyratory equipment Mechanical screening Molecular sieve Separation process Sieve analysis Soil gradation Filter
A Turbo mixer known as a high speed mixer or a tank mixer, is a type of industrial mixer that uses PVC for mixing raw materials to form a free-flowing powder blend. It includes a cylindrical tank with a mixing tool assembled on the bottom that operates at a peripheral speed of between 20 and 50 m/s, depending on the material to blend; the material is heated inside by a mixer, through the mechanical energy, produced between the mixing tools and the material which generates mutual impacts of the particles. During the mixing phase, the Turbo-mixer creates an axial vortex; the structure and position of the blades inside the mixer guarantee homogeneous material dispersion. To avoid thermal degradation, it is combined with a cooler that cools down the dry blend to the temperature of around 45-55 C. Due to the poor heat conductivity of the cooler, the cooler is three times larger than the mixer as the cooling time is proportional to contact surface; the typical uses of the Turbo mixer is for the production of PVC and for other kinds of thermoplastic composites.
The largest high-speed mixer known on the market has a tank volume of 2500 litres, which corresponds to a PVC batch size of about 1060 l and is combined with a horizontal cooler 8600 l. It was manufactured by the Italian company PROMISING S.r. L. in 2014
Medium-density fibreboard is an engineered wood product made by breaking down hardwood or softwood residuals into wood fibres in a defibrator, combining it with wax and a resin binder, forming panels by applying high temperature and pressure. MDF is denser than plywood, it is made up of separated fibres, but can be used as a building material similar in application to plywood. It is much denser than particle board; the name derives from the distinction in densities of fibreboard. Large-scale production of MDF began in both North America and Europe. Over time, the term MDF has become a generic name for any dry process fibre board. MDF is made up of 82% wood fibre, 9% urea-formaldehyde resin glue, 8% water and 1% paraffin wax. and the density is between 500 kg/m3 and 1,000 kg/m3. The range of density and classification as light, standard, or high density board is a misnomer and confusing; the density of the board, when evaluated in relation to the density of the fibre that goes into making the panel, is important.
A thick MDF panel at a density of 700–720 kg/m3 may be considered as high density in the case of softwood fibre panels, whereas a panel of the same density made of hard wood fibres is not regarded as so. The evolution of the various types of MDF has been driven by differing need for specific applications. There are different kinds of MDF: Ultralight MDF plate Moisture resistant is green Fire retardant MDF is red or blueAlthough similar manufacturing processes are used in making all types of fibreboard, MDF has a typical density of 600–800 kg/m³ or 0.022–0.029 lb/in3, in contrast to particle board and to high-density fibreboard. In Australia and New Zealand, the main species of tree used for MDF is plantation-grown radiata pine. Where moisture resistance is desired, a proportion of eucalypt species may be used, making use of the endemic oil content of such trees; the trees are debarked after being cut. The bark can be burned in on-site furnaces; the debarked logs are sent to the MDF plant. A typical disk chipper contains 4–16 blades.
Any resulting chips that are too large may be re-chipped. The chips are washed and checked for defects. Chips may be stored as a reserve for manufacturing. Compared to other fibre boards, such as Masonite, MDF is characterised by the next part of the process, how the fibres are processed as individual, but intact and vessels, manufactured through a dry process; the chips are compacted into small plugs using a screw feeder, heated for 30–120 seconds to soften the lignin in the wood fed into a defibrator. A typical defibrator comprises two counter-rotating discs with grooves in their faces. Chips are fed outwards between the discs by centrifugal force; the decreasing size of the grooves separates the fibres, aided by the softened lignin between them. From the defibrator, the pulp enters a distinctive part of the MDF process; this is an expanding circular pipeline 40 mm in diameter, increasing to 1500 mm. Wax is injected in the first stage, which coats the fibres and is distributed evenly by the turbulent movement of the fibres.
A urea-formaldehyde resin is injected as the main bonding agent. The wax improves moisture resistance and the resin helps reduce clumping; the material dries in the final heated expansion chamber of the blowline and expands into a fine and lightweight fibre. This fibre may be used or stored. Dry fibre gets sucked into the top of a'pendistor', which evenly distributes fibre into a uniform mat below it of 230–610 mm thickness; the mat is pre-compressed and either sent straight to a continuous hot press or cut into large sheets for a multi-opening hot press. The hot press sets the strength and density profile; the pressing cycle operates in stages, with the mat thickness being first compressed to around 1.5× the finished board thickness compressed further in stages and held for a short period. This gives a board profile with zones of increased density, thus mechanical strength, near the two faces of the board and a less dense core. After pressing, MDF is cooled in a star dryer or cooling carousel and sanded.
In certain applications, boards are laminated for extra strength. The environmental impact of MDF has improved over the years. Today, many MDF boards are made from a variety of materials; these include other woods, recycled paper, carbon fibres and polymers, forest thinnings and sawmill off-cuts. As manufacturers are being pressured to come up with greener products, they have started testing and using non-toxic binders. New raw materials are being introduced. Straw and bamboo are becoming popular fibres. MDF does not contain knots or rings, making it more uniform than natural woods during cutting and in service. However, MDF is not isotropic, since the fibres are pressed together through the sheet. Typical MDF has a hard, smooth surface that makes it ideal for veneering, as there is no underlying grain to telegraph through the thin veneer as with plywood. A so-called "Premium" MDF is available that features more uniform density throughout the thickness of the panel. MDF may be doweled or laminated.
Typical fasteners are pan-head machine screws. Smooth-shank nails do not hold well, neither do fine-pitch screws, especi
Mixing (process engineering)
In industrial process engineering, mixing is a unit operation that involves manipulation of a heterogeneous physical system with the intent to make it more homogeneous. Familiar examples include pumping of the water in a swimming pool to homogenize the water temperature, the stirring of pancake batter to eliminate lumps. Mixing is performed to allow heat and/or mass transfer to occur between one or more streams, components or phases. Modern industrial processing always involves some form of mixing; some classes of chemical reactors are mixers. With the right equipment, it is possible to mix a solid, liquid or gas into another solid, liquid or gas. A biofuel fermenter may require the mixing of microbes and liquid medium for optimal yield; the opposite of mixing is segregation. A classical example of segregation is the brazil nut effect; the type of operation and equipment used during mixing depends on the state of materials being mixed and the miscibility of the materials being processed. In this context, the act of mixing may be synonymous with stirring kneading-processes.
Mixing of liquids occurs in process engineering. The nature of liquids to blend determines the equipment used. Single-phase blending tends to involve low-shear, high-flow mixers to cause liquid engulfment, while multi-phase mixing requires the use of high-shear, low-flow mixers to create droplets of one liquid in laminar, turbulent or transitional flow regimes, depending on the Reynolds number of the flow. Turbulent or transitional mixing is conducted with turbines or impellers. Mixing of liquids that are miscible or at least soluble in each other occurs in process engineering. An everyday example would be the addition of milk or cream to coffee. Since both liquids are water-based, they dissolve in one another; the momentum of the liquid being added is sometimes enough to cause enough turbulence to mix the two, since the viscosity of both liquids is low. If necessary, a spoon or paddle could be used to complete the mixing process. Blending in a more viscous liquid, such as honey, requires more mixing power per unit volume to achieve the same homogeneity in the same amount of time.
Blending powders is one of the oldest unit-operations in the solids handling industries. For many decades powder blending has been used just to homogenize bulk materials. Many different machines have been designed to handle materials with various bulk solids properties. On the basis of the practical experience gained with these different machines, engineering knowledge has been developed to construct reliable equipment and to predict scale-up and mixing behavior. Nowadays the same mixing technologies are used for many more applications: to improve product quality, to coat particles, to fuse materials, to wet, to disperse in liquid, to agglomerate, to alter functional material properties, etc; this wide range of applications of mixing equipment requires a high level of knowledge, long time experience and extended test facilities to come to the optimal selection of equipment and processes.. Solids-solids mixing can be performed either in batch mixers, the simpler form of mixing, or in certain cases in continuous dry-mix, more complex but which provide interesting advantages in terms of segregation and validation.
One example of a solid–solid mixing process is mulling foundry molding sand, where sand, bentonite clay, fine coal dust and water are mixed to a plastic and reusable mass, applied for molding and pouring molten metal to obtain sand castings that are metallic parts for automobile, machine building, construction or other industries. In powder two different dimensions in the mixing process can be determined: convective mixing and intensive mixing. In the case of convective mixing material in the mixer is transported from one location to another; this type of mixing leads to a less ordered state inside the mixer, the components that must be mixed are distributed over the other components. With progressing time the mixture becomes more randomly ordered. After a certain mixing time the ultimate random state is reached; this type of mixing is applied for free-flowing and coarse materials. Possible threats during macro mixing is the de-mixing of the components, since differences in size, shape or density of the different particles can lead to segregation.
When materials are cohesive, the case with e.g. fine particles and with wet material, convective mixing is no longer sufficient to obtain a randomly ordered mixture. The relative strong inter-particle forces form lumps, which are not broken up by the mild transportation forces in the convective mixer. To decrease the lump size additional forces are necessary; these additional forces can either be impact forces or shear forces. Liquid–solid mixing is done to suspend coarse free-flowing solids, or to break up lumps of fine agglomerated solids. An example of the former is the mixing granulated sugar into water. In the first case, the particles can be lifted into suspension by bulk motion of the fluid. One example of a solid–liquid mixing process in industry is concrete mixing, where cement, small stones or gravel and water are co
The glass–liquid transition, or glass transition, is the gradual and reversible transition in amorphous materials, from a hard and brittle "glassy" state into a viscous or rubbery state as the temperature is increased. An amorphous solid that exhibits a glass transition is called a glass; the reverse transition, achieved by supercooling a viscous liquid into the glass state, is called vitrification. The glass-transition temperature Tg of a material characterizes the range of temperatures over which this glass transition occurs, it is always lower than the melting temperature, Tm, of the crystalline state of the material, if one exists. Hard plastics like polystyrene and poly are used well below their glass transition temperatures, i.e. when they are in their glassy state. Their Tg values are well above room temperature, both at around 100 °C. Rubber elastomers like polyisoprene and polyisobutylene are used above their Tg, that is, in the rubbery state, where they are soft and flexible. Despite the change in the physical properties of a material through its glass transition, the transition is not considered a phase transition.
Such conventions include a constant cooling rate and a viscosity threshold of 1012 Pa·s, among others. Upon cooling or heating through this glass-transition range, the material exhibits a smooth step in the thermal-expansion coefficient and in the specific heat, with the location of these effects again being dependent on the history of the material; the question of whether some phase transition underlies the glass transition is a matter of continuing research. The glass transition of a liquid to a solid-like state may occur with either compression; the transition comprises a smooth increase in the viscosity of a material by as much as 17 orders of magnitude within a temperature range of 500 K without any pronounced change in material structure. The consequence of this dramatic increase is a glass exhibiting solid-like mechanical properties on the timescale of practical observation; this transition is in contrast to the freezing or crystallization transition, a first-order phase transition in the Ehrenfest classification and involves discontinuities in thermodynamic and dynamic properties such as volume and viscosity.
In many materials that undergo a freezing transition, rapid cooling will avoid this phase transition and instead result in a glass transition at some lower temperature. Other materials, such as many polymers, lack a well defined crystalline state and form glasses upon slow cooling or compression; the tendency for a material to form a glass while quenched is called glass forming ability. This ability can be predicted by the rigidity theory. Below the transition temperature range, the glassy structure does not relax in accordance with the cooling rate used; the expansion coefficient for the glassy state is equivalent to that of the crystalline solid. If slower cooling rates are used, the increased time for structural relaxation to occur may result in a higher density glass product. By annealing the glass structure in time approaches an equilibrium density corresponding to the supercooled liquid at this same temperature. Tg is located at the intersection between the cooling curve for the glassy state and the supercooled liquid.
The configuration of the glass in this temperature range changes with time towards the equilibrium structure. The principle of the minimization of the Gibbs free energy provides the thermodynamic driving force necessary for the eventual change, it should be noted here that at somewhat higher temperatures than Tg, the structure corresponding to equilibrium at any temperature is achieved quite rapidly. In contrast, at lower temperatures, the configuration of the glass remains sensibly stable over extended periods of time. Thus, the liquid-glass transition is not a transition between states of thermodynamic equilibrium, it is believed that the true equilibrium state is always crystalline. Glass is believed to exist in a kinetically locked state, its entropy, so on, depend on the thermal history. Therefore, the glass transition is a dynamic phenomenon. Time and temperature are interchangeable quantities when dealing with glasses, a fact expressed in the time–temperature superposition principle. On cooling a liquid, internal degrees of freedom successively fall out of equilibrium.
However, there is a longstanding debate whether there is an underlying second-order phase transition in the hypothetical limit of infinitely long relaxation times. Refer to the figure on the upper right plotting the heat capacity as a function of temperature. In this context, Tg is the temperature corresponding to point A on the curve; the linear sections below and above Tg are colored green. Tg is the temperature at the intersection of the red regression lines. Different operational definitions of the glass transition temperature Tg are in use, several of them are endorsed as accepted scientific standards. All definitions are arbitrary, all yield different numeric results: at best, values of Tg for a given substance agree within a few kelvins. One definition refers to the viscosity; as evidenced experimentally, this value is close to the annealing point of many glasses. In contrast to viscosity, the thermal expansion, heat capaci