Plastic Product Material and Process Selection Handbook Part 12 doc

CALENDERING Introduction This process is used to convert thermoplastic materials into continuous sheets, films, and for applying plastic coatings to textiles, paper, or other supporting

CALENDERING

Introduction

This process is used to convert thermoplastic materials into continuous sheets, films, and for applying plastic coatings to textiles, paper, or other supporting material When coating the calendering line is also called a coating machine Calendering is an alternative to extrusion with the usual film at three or more mils (75 microns) thick (Chapter 5) For the production of sheet or film plastic melt is compounded and pressed as it passes through the nips of a series of three or more heated highly polished steel rolls

A plastic bank is formed into a web in the nip between the first pair of rolls Passing through the second and third nips further reduces the thickness Final thickness of the sheet is determined by the gap between the last pair of rolls called the gauging rolls Finally, a take-off roll pulls the hot sheet around a chilled roll to cool the sheet or film web (Figure 9.1) In this industry bank is identified as the quantity of plastic present

in the nip formed between two rolls (Figure 9.1) [Bank marks are surface roughness on sheet caused by incorrect temperature or sizes of banks They can be minimized by optimizing formulations, calendering speeds, and roll temperatures so as to obtain the most orderly behavior

of the rolling banks of stock at the calender-nip entrances.]

Calendering converts plastic into a melt and then passes the pastclike melt through roll nips of a series of heated and corotating speed- controlled rolls into wcbs of specific thickness and width The web may

be polished or embossed, either rigid or flcxiblc Proper calendering rcquircs precise control of the complete roll tcmpcraturcs, pressures, and specd of rotations An cmbosscd design can be produced on the surface by using an engraved roll, calendering a mixture of granular

3 7 0 Plastic Product Material and Process Selection Handbook

Figure 9.I Example of the sheet or film passing through nip rolls to decrease thickness

plastic chips of varying color may produce unusual decorative effects such as marblization, and so forth Calendering often processes vinyl plastics

The complete equipment usually consists of a mixer such as a Banbury mixer followed by the heated rolls, chilled rolls, and finally a windup roll 3 The windup roll controls the tension on the film or sheeting as it moves through the calender rolls Calenders arc generally designed to meet the specific needs of the customer Once installed and operating continuously, the cost per pound of film or sheet is lower than by any other process such as extrusion

The capital cost for a calendering line will average at least $10 million

A line, probably the largest in the world processing PVC sheet was build by Kleinewefers Kunststoffanlagen GmbH, Munich, Germany Cost for this 5 roll L-type configuration was $33 million (1999) It has 3,500 mm roll-face widths and 770 mm diameters with an output rate

at 4,000 k g / h

Plastics that melt to a rather low viscosity are not suitable for calendering Additives can have a major influence on processability With this understanding comes the ability to make calenders more productive by increasing their speed They also produce films and

sheets with tighter thiclmess tolerances and improved uniformity and can handle thicker sheets more effectively

of equipment may not be in the storeroom From the start to the end

of the calendering process extreme care has to be taken to ensure there

is no contamination of the equipment or plastic being processed Preventative maintenance of these lines is a continuous operation that includes the operating environment in the plant to be a relatively clean room

Calenders vary in respect to the number of rolls and their arrange- ments Examples of the layout of the rolls are the true L, conventional inverted L, revcrsc fed inverted L, 1, Z, and so on These large diameter heated rolls have the function to convert the high viscosity plastic melt into film or sheet Figures 9.2 and 9.3 provide examples of lines

Figure 9,2 Calender line starting with mixer

In the early days of calendering plastics three-roll vertical rubber machines were used Problems developed in processing plastics They

372 Plastic Product Material and Process Selection Handbook

Figure 9.3 Examples of the arrangements of rolls in a calender line

included difficulty in feeding horizontal nip, gauge variations, temperature variations due to using cored rolls, no capability for cross- axis or roll bending adjustments, and roll floating due to pressure variations in the feed nips As time passed these problems were continually reduced or eliminated particularly on the smaller calenders The offset rolls were designed to eliminate the major difficulty of the horizontal feed nip Because the material drops by gravity into the vertical pass, the offset feed nip provides important savings in manpower and yield Mso, the pressure fluctuations of the feed to the other nips are minimized because roll No 2 will tend to float horizontally rather than vertically in relation to roll No 3 (Figure 9.3)

To reduce gauge variation in this setup fitting roller bearings can stabilize roll No 3 floating roll Cross-axis a n d / o r roll bending may be fitted to roll No 3 or roll No 4 With this compact setup it is still easily accessible for starting up and operating the machine

The Z-type roll arrangements followed developments in offset rolls This design eliminated the floating No 3 roll on a calender fitted with bearings Each roll can be preloaded on to its bearings at a point that is

the resultant of the material pressures and the roll weight This approach had other advantages that included reduction of the height required for the installation of rolls In turn plant space requirement was reduced along with reduced building cost Its disadvantage is limiting the ease of access to roll No 2 or No 3 in the case of the inverted Z With the inclined Z it is more difficult to feed than a standard type Z because the nip does not hold as much material

Calenders with at least four to six rolls are used to fabricate thin rigid sheet where the extra nips greatly improve the surface finish of the sheet The more popular are the four-roll inverted L calender and Z calender The Z calender has the advantage of lower heat loss in the film or sheet because of the melts shorter travel and the machines' simpler construction They are simpler to construct because they need less compensation for roll bending This compensation occurs because there arc no more than two rolls in any vertical direction as opposed to three rolls in a four roll inverted L type calender The speed of the calendering rolls usually differs They operate at different speeds to provide the best performance of the melt, particularly the required shearing action (Chapter 1)

High pressures of at least up to 6,000 psi (40 MPa) can bend or deflect the rolls This calender bowl deflection is the distortion suffered by calender rolls resulting from the pressure of the plastic running between them If not corrected, the deflection produces film or sheets thicker in the middle than at the edges The amount of thrust exerted by the material depends on processing factors such as method of feeding stock into the calender, plastic temperature, melt flow behavior (Chapter 1), required thickness and width, and speed of the calendering line Unfortunately the rolls do not bend like a simple beam that is freely supported at each end and uniformly loaded along its length Each calender roll varies in thickness between the face and its journal Because it rotates the pressure distribution across the roll is not exactly equal Thus it does not deflect on conformation with the classical engineering equation 1 but in such a manner simulating a profile of a U-shaped frame forming a collar about an ox's neck resembling an oxbow

In order to compensate for this thicl~ess variation requires the surface

of the roll to fit a certain profile (crown) The amount of crown, that is the difference in roll section radius between ends and center, will vary depending on the rhcological properties of the plastic being processed (Chapter 1) Rolls arc crowned resulting in having a greater diameter in the middle The equipment also provides for different types of adjust- mcnts and controls (crossing of rolls and roll bending) to correct

3 7 4 Plastic Product Material and Process Selection Handbook

distortion Example is crossing the rolls slightly rather than having them truly parallel; results in increasing the nip opening at both ends of the roll Less deflection at high operating conditions can be achieved by the use of stiffer rolls, based on higher modulus of elasticity steels or dual-steel construction Another approach is to bend the roll so that the bending moment is applied to the end of each roll by having a second bearing on each roll neck In turn a hydraulic cylinder loads it

Calenders require high temperatures with little variations or fluctuations across the rolls during the application of the high pressures

on the stock Flow of stock relates to the friction between the stock and the roll faces, stock viscoelasticity, and pressure applied on the plastic The first matching rolls provide initial control feeding plastic into the calender system The final matching rolls provide the final roll thickness control of the sheet or film Those matching rolls in between provide a gradual thickness metering action Adjusting roll temperatures and speeds controls the final product dimensions Roll loads run 1000 to

2000 Ib/linear in of roll face for soft sheeting, and occasionally approach 5000 lb/linear in for thin, rigid material processed cool at 330F (166C) on larger rolls Total connected horsepower can run from

2 yd./min, on 24 in calenders, to as much as 8 to 10 for a large 36 by

96 in machine on tough plastics

Any unevenness in the temperature and pressure along the roll's length, that could include uneven temperature across the melt, is reflected as variations in the product thickness Other causes of thickness changes across the web include nonhomogeneous rheology of the stock (Chapter 1 ), problems with material's lubricity, malfunctioning pressure and temperature sensors, equipment line control malfunctioning, use of damaged calender rolls, and so on Also critical is the cooling of film or sheet that use multiple water-cooled rolls in the calender line with roll temperatures gradually reduced as the plastic travels downstream The sheet or film immediately passes through precision surfaced cooling rolls that are kept at precisely controlled temperatures a n d / o r a cooling tower where the web can be festooned At least two to ten to possibly 20 cooling rolls are used depending on the thickness of web and the speed of production line With more cooling rolls the line permits slower cooling to room temperature eliminating a shock cooling situation for certain plastics that reduces physical and mechanical properties such as rigid PVC If embossing is to be applied, the embossing roll precedes these cooling rolls After leaving the last large diameter calendering heated rolls, the film can be literally dropped vertically into an embosser, usually with three r o l l s - that is the embossing roll itself, a cooling rubber roll, and a contact cooling to the

rubber roll Temperature accuracy is usually controlled within + I C (e2F)

Since the heated plastic clings to the calender rolls the web does not drop off the last roll It has to be pulled off evenly across the width of the roll This is accomplished by the stripper roll which is normally positioned 3 to 6 in (75 to 150 mm) from the last roll, and at a height that gives the sheet approximately 270 ~ lap round the roll

Overall after the heated plastic passes through the rolls it can go through operations of stripping, embossing, cooling, trimming, and wind-up Because here the hot plastic is in contact with a comparatively cold roller, for PVC there may be a problem of plasticizer and moisture condensing on the metal surface of the stripper roll This condensate will mark or, in the case of condensed plasticizer, attack the sheet surface To overcome this damaging action the stripper roll is covered with a highly absorbent material such as cloth

The thinner the sheet the greater the degree of roll cling, Thus the speed of the stripper roll must be varied with respect to the calender speed Once the desired speed differential is set it is maintained As the calender speed is altered, the stripper roll speed maintains a constant ratio with the calender speed

Different types of controls arc available to meet specific operating conditions (Chapter 3) Propcr use of all controls is required to meet product performances and minimize costs The controls can call for adjustments on different line equipment, such as the nip openings, roll bending, neckdown, and so on As an example proper use of ncckdown roll permits windups to bc run faster than the final calender roll on many thin, unsupported film products Calenders and rake-offs arc run almost synchronously on heavy gauge products Films and sheets with a high gloss taken off a highly polished final calender roll tend to stick to the roll more than their matte counterparts Very soft webs also tend to stick to the final calender roll The fastest calender speeds arc generally obtained in a median thiclmess range

Trimming can be performed either on the calender or later when the sheet is cold just prior to winding It is economically sound to trim at the calender stage where the material, owing to its existing temperature, can

be readily conveyed back to the calender feed nip, to a set of rolls, an extruder feeder for recycling, or a granulator and blended with virgin plastic Following cooling the plastic can bc trimmed at the edges and wound Trim material can account for up to 5% of the width depending

on the line's operating efficiency The target is to have as little trim as possible This operation is to cool the sheet to ambient temperatures If

3 7 6 Plastic Product Material and Process Selection Handbook

warm or hot sheet is wound up, high internal strains may be caused and blocking and de-embossing problems may be introduced Ideally, sheet should be wound up at approximately I OC (5OF)

Wind-up occurs at the end of the line The two usual methods of winding into rolls are center-core winding and surface batching Not all calendered sheets are wound up into rolls They are also cut into panels

by rotary cutters or automatic guillotines that may be installed instead

of wind-up equipment With center-core winding one end of the mandrel is fitted into a socket which is power driven It requires that uniform sheet or film tension is used or the product will not be uniform

in thickness, etc As the roll increases in size the moment of inertia builds up and the take-up force per revolution increases Unless the drive can compensate for this force increase, the winding tension varies throughout the roll By appropriately adjusting the tensions, winding can be applied to rigid or flexible plastics Methods used to overcome this tension situation include a slipping clutch between the mandrel and the drive, or more usually, having the drive to the mandrel transmitted

by a motor drive This action controls the sheet tension at a predeter- mined value regardless of the increasing diameter as the roll winds up

To facilitate roll changing the winding station is usually duplicated, thus allowing one roll to wind while the other is being removed Other auxiliary equipment can be included in the line such as orienting by stretching in the machine direction a n d / o r transverse direction using the cooling rolls or setup bioriented stretching (Chapters 5 and 18), annealing, decorating, slitting, heat sealing, festooning, and so on

Corn pou nd i ng/B lending

Different plastics, each with variations in type and quantity of additives, fillers a n d / o r reinforcements, result in providing different processing conditions and end product performances Important is the proper preparation of the plastic compounded stock to be processed based on weight as well as order of mixing Stock prepared effects factors such as how the calender is to be operated, take-off thickness measurements, windup system requirements, and line speed controls Other factors that influence the preparation of a stock is related to the finish (glossy, semi-matte, matte, etc.), product requiring coating or laminated to a substraight (fabric, plastic film or sheet, aluminum foil, etc.), embossed, etc.), or include if web is slit in line With the finished product special properties may be required such as optical clarity and mono or biaxial orientation (Chapter 5)

Blending or compounding of the plastic with different additives and fillers is a critical part of the process, particularly of PVCs The PVC compounds require heat stabilizers in order to be properly processed Heat stabilizer system imparts during processing primarily heat stability,

as well as adequate lubricating characteristics to reduce or control frictional heat Stabilizers are also very efficient for plate-out resistance Plate-out is a condition where the calender rolls a n d / o r embossing rolls become coated with a deposit from the compound being processed that

in turn interferes with obtaining an acceptable surface finish of the film

or sheet This deposit may start out as a soft, waxy material barely visible on the metallic contact surfaces of the processing equipment When plate-out occurs the line has to be shut down and the contamination removed

Processing

Because the plastic is processed between the required heat and its critical heat of degradation, the time of heat becomes extremely critical and an important part of the complete process For example the processor will minimize the amount of melt in the nip of the rolls The residence time of the plastic flux at high heat must be controlled and limited PVC is especially sensitivity to heat and time at heat What is required is proper setting of the machine controls and operation within set limits The processing variables of a PVC plastic (such as flow, heat stability and softening point) are strongly influenced by polymerization technique, MWD, and the extent of any polymerization (Chapter 1)

Due to the plastic's viscosity, a melt shear effect is developed throughout the process This shear is of prime importance between the calender rolls The calender forms the web as a continuous extrusion between the rolls (Chapter 1) Unlike when processing just through a conventional extrusion line, the plastic mass cannot be confined when being calendered Because of the lack of confinement, the shear effect and a broad melt band are essential aspects of calendering

TO improve PVC melt flow the stock is subjected to fluxing or fusion It

is the heating of the vinyl compound to produce a homogeneous mixture Fluxing units used in calendering lines include batch-type Banbury mixers, Farrel continuous mixers (FCMs), Buss Ko-Kneaders (BKKs), and planetary gear extruders (PGEs) The dry blend is fed into the mixer/extruder Proper mixing within a short dwell time and heat transfer control contributes to an improved product During fluxing, each particle receives the same gentle treatment, generating less heat

378 Plastic Product Material and Process Selection Handbook

history and producing more uniform feed rate, color, gauge thickness, web surface, and so on The feed can discharge onto a two-roll mill Operating this way, it provides for a second fluxing action, mainly for working in scrap or for convenience as a buffer

Rigid PVC manufacturers usual prefer the L-type with four to seven rolls being fed from the floor level Since there is no disturbing vapors from lower calender rolls within the pickoff area, it is preferable to have the pickoff rolls on an elevated level Flexible PVC is commonly processed using a 4-roll inverted L- or an F-type A universal five roll L calender is used for rigid or flexible PVC film It provides heat stability and superior film control with good surface appearance The major difference between this universal machine and the others is in mounting and placement of the first roll These systems enable the plasticizer- saturated vapors to escape via the usual suction hood located above the calender where they are filtered before being released to the atmosphere The stock delivered to the first calender nip needs to be well fused, homogeneous in composition, and relatively uniform in temperature The optimum average temperature for good fusion depends on the formulation A rigid PVC formulation based on medium molecular weight plastic (intrinsic viscosity of 0.90 to 1.15)211 has a typical optimum temperature of 180 to 190C (355 to 375F) at the first calender nip For best calendering, there should be no cold volume elements below 180C (356F) and no hot spots above 200C (392F) Required is close control of temperature to ensure proper fusion and mixing conditions

This interaction depends on stock temperature and in turn on the performance of PVC melts Flexible PVC is normally calendered at temperatures of 10 to 20C (50 to 68F) lower than rigid PVC In flexible PVC production, a short single screw extruder acting as a strainer filters out contaminants from stock before reaching the calender This important method is not applicable to rigid PVC because

it drastically increases the head pressure and the consequent overheating would cause the stock to decompose

Market

Products from calenders go into many different markets such as credit cards, upholstery, luggage, water reservoir, rainwear, loose-leaf book, and footwear Different plastics are used such as ABS and ABS/PVC alloys go into margarine pack, luggage, panels, and chlorinated PE go into roofing, and pond liners There are unsupported and supported as well as rigid products and coated substrates Unsupported flexible PVC

is in label tapes, flooring tiles, pool liners, crop covers, raingears, tank linings, packaging liquids, shower curtains, auto interiors and trims, ditch linings, book binders, electrical and pipe wrap tapes, auto crash pads, inflatables (such as air beds, swim rings, and children's paddling pools), headliners, mattress covers, crib linings, baby pants, convertible rear windows, hand bags, moisture barriers, chemical resistant panels, and pressure-sensitive adhesives

Supported rigid PVC is in window shades, wall and floor coverings, tablecloths, woodgrain laminations, book liners, and labels Rigid PVC

is in hardwares and food packs, trays, pharmaceutical packs, credit cards, lighting fixtures, ceiling tile facings, woodgrains, laminate covers, signs, tank linings, corrosive duct works, thin tapes, strapping tapes, trays, helmet liners, and printers' products

Coated substrates involve different materials such as coated credit cards, paper, woven and nonwoven textiles, plastic or aluminum films and sheets, and roll coverings Calender lines can process one coated side, both sides, or laminated (multiple substrates coated between each substrate) Calender with three rolls is usually sufficient for one-sided coating However four rolls are used for extremely thin coatings The 4-roll calender can be used for double-sided coating that is applied simultaneously on both sides Specialized calendering equipment is used for certain products such as credit cards, floor tiles, and window curtains

The application of flexible sheet material to the surface of mandrels, called roll covering, is used in a variety of industries that include printing, paper, textiles, steel, office machinery, plastic fabricating lines, and many others (Figure 9.4) Their use includes to compress, drive, emboss, convey, protect, dye, suction, treat, piclde, paint, and print

Calendering vs Extrusion

Calendering and extrusion lines (Chapter 5) produce film, sheet, and for applying plastic coatings to textiles, paper, or other supporting material Table 9.1 provides comparison in fabricating PVC film The extrusion process provides flexibility, when compared to calendering, that includes ease of changing product thicknesses, widths, materials, and provides for short production runs

Calendered sheet is usually less glossy than extruded material Calendering may be preferable for certain applications requiring its higher tensile properties, product uniformity, and unusually close gauge

3 8 0 Plastic Product Material and Process Selection Handbook

Figure 9.4 Example of roll covering

Relative resin cost lowest

Machine cost ($ million) 1- I0

Rate and range (lb h -~) 800-8000

Product gauge range (in) 0.002-0.050

Sheet accuracy (%) 3 (1-5)

Time to heat (h) 6

Thne tbr s "~wtup 2-5 rain

Gauge adjust time seconds

Autogauging, capability yes

Color or product 5-30 rain

0.OO2-0.OO5

3(1-5)

5

10 min seconds yes 10-40 rain

60 (80)

Lower rate, versatility problem

Extruder Extruder Blown fdm flex-lip higher higher 0.3- I 0.3 1

min 30-60 rain

lS (20) 15 (30) Poor accuracy, long on startup time, low rate, degradation, reduced versatility

Applications and Versatility, high

advantages rate, accuracy.,

ease and, adjustment ease

at reprocess

i

Accuracy, gauge adjust, reduced cost

Low investment, multiplant capability,

heavy gauge (0,050-0.125 in)

control Extrusion of colored films or sheets requires the extruder to bc cleaned and purged when changing colors A calender requires a minimum of cleaning between color changes Calendering definitely has to be used for long production runs in order to be economically profitable, producing smooth and other finishes at higher speeds In general, plastic materials, such as PE, PP, and PS film and sheet, are

usually produced through the rather conventional extrusion lines To produce PVC film and sheet in large quantities, calendering is almost always used since the process is less likely to cause degradation than is extrusion as well as having dimensional and cost advantages

The capital equipment and replacement parts in calendering lines are more expensive A web thickness between 0.05 to 0.50 mm (0.002 to 0.020 in.) is generally the kind of plasticized film and sheeting produced by calender lines For extremely light gauges, those under 0.02 mm (0.001 in.), calendering could become impractical or damaging to the equipment The reasons include factors such as, for certain materials, there exists poor strength of the thin webs and also very high forces develop on the matting heavy-duty rolls H e a w / t h i c k gauges, such as sheeting over 0.50 mm (0.020 in.), calendering may not be the optimum method of production The reason is that there may not be enough shearing action that can be put into the rolling banks to keep the compound at uniform temperature In addition, the separating forces on the rolls become so low that gauges variations could become prohibitive

In summarizing the productivity of calendering the type of calendered product is significant Hea W sheeting, the easiest product to make can run at high speeds, depending on fluxing and feeding capacity If the product is post-treated with laminating to a substrate, embossed, printed, or top-coated, production can be even greater since defects in the sheet can be masked

Thin flexible film, sold straight off the calender, is difficult to make because of layflat problems, although speeds of 100 y d / m i n , at the calender and 125 yd./min, at the winder are common Some post- treated rigid films can run at 80 yd./min., but other rigid sheets of the glossy or polished variety are limited to about 20 to 35 yd./min, for top quality Thus, the rates through a line may range from a low of 800

to a high approaching at least 9000 lb./hr

The main disadvantages of calendering are large initial investment costs and lengthy heat up times The advantages that make the calender ultimately the most desirable method of all are maximum rates and speeds, accuracy of gauge, speed of gauge adjustment, processing and product range versatility, lower raw plastic costs, high on-stream time factors, fast on-line time, and case of accommodation of automatic gauging and control

COATI N G

Overview

Coated products using thermoplastics (TP) and thcrmoset plastics (TS) are literally all around us worldwide This large industry produces two broad categories of coatings, namely, the trade sales and the industrial finishes Trade sales, or shelf goods, include products sold directly to consumers, contractors, and professional painters for use on construction

or painting, refinishing, and general maintenance 261 These coatings are used chiefly on houses and buildings, although a sizeable portion is used for refinishing automobiles and machinery Also included are electric/electronic, packaging, building, household and industrial appliances, transportation, marine, medical, 474 clothing, and many more

Industrial finishes, or chemical coatings, encompass a myriad of products for application by manufacturers in the factory or for industrial maintenance and protection They are custom made products sold to other manufacturers for such items as automobiles, appliances, furniture, ships and boats, metal containers, streets and highways, and government facilities

Coating compounds are used to cover the surfaces of many materials from plastic to paper to fabric to metal to concrete and so on Many plastics produced are consumed as coating materials, including paints, primers, varnishes, and enamels Metals may be surface coated to improve their workability in mechanical processing Substrates protected from different environmental conditions basically include the metals (steel, zinc, aluminum, and copper), inorganic materials (plaster, concrete, and asbestos) and organic materials (wood, wallboard, wallpaper, and plastics) Different technical developments continue to occur in the

coating industry, which permit the use of a variety of plastics It is possible to formulate surface coatings that are suitable for each and every kind of material

Type

Coatings are generally identified as paints, lacquers, varnishes, enamels, hot melts, plastisols, organosols, water-emulsion, solution finishes, nonaqueous dispersions, powder coatings, masonry water repellents, polishes, magnetic tape coatings, overlays, gels, compound, etc Paint and some of the other coatings may be identified as interior or exterior type 262 Each type usually has its own identification such as the lacquer coating is a cellulosic composition that dries by the evaporation of the solvent Varnish identifies a mixture of plastic and oil The term paint is often used to cover all the coating categories as though it was synonymous with coating; the terms are often used interchangeably Paint coatings consume by far the largest quantity of coating material However the other coating processes are important and useful All these surface coatings represent a large segment of the overall plastic and chemical industries

There are 100% resin coatings such as vinyl-coated fabrics or polyure- thane floor coverings The usual components of paint and other coatings are the binder (resin), pigment, solvent, and additive The binder provides the cohesive forces that hold the film together and holds the coating film to the substratr The pigment that is in a fine powder provides color and properties such as hardeners and resistance

to abrasion and weathering The pigment has a considerable influence

on the consistency (viscosity) of the paint and in turn on its application properties The volatile liquid solvent provides the means to dissolve the binder Coating systems may contain additives to meet certain processing a n d / o r performance requirements Examples are stabilizers, plasticizers, dryers, wetting agents, flattening agents, and emulsifiers

The binder is the most important of the components and is always present in a manufactured paint It usually represents 40 to 50wt% of the paint Many of the properties of paints and related products are determined directly by the nature of the binder For this reason paints are often classified and may even be named according to the type of binder Binders are identified according to type of drying The physical and chemical drying types relate to how they are formulated The physical film type results in the evaporation of the solvent or of dispersion medium in the case of paint lattices Chemical film type has

3 8 4 Plastic Product Material and Process Selection Handbook

an oxidative drying constituent such as drying oils, varnishes, linseed oil, tung oil, and alkyd plastic modified with drying oils

Coating vehicle usually identifies a combination of binder and volatile liquid It may be a solution or a dispersion of fine binder particles in a nonsolvent formulation No pigments are included if a clear, transparent coating is required The composition of the volatile liquid provides enough viscosity for packaging and other application, but the liquid itself rarely becomes part of the finished coating

Film coating can involve chemical reaction, polymerization, or cross- linking Some films only involve coalescence of plastic particles There are various mechanisms involved in the formation of plastic coatings They can be identified as follows:

(a) dispersions of a plastic in a vehicle followed by removal of the

Vehicle via evaporation or heat baking; result is the plastic coalesces

to form a film of plastisol, organosol, water-based, or latex paint;

(b) pigments in oil that polymerizes in the presence of oxygen and drying agents that include alkyd, enamels, and varnishes;

(c) coating formed by chemical reaction, polymerization or crosslinking

of TS plastics;

(d) plastic dissolved in a solvent followed by solvent evaporation to leave a plastic film of vinyl lacquer, acrylic lacquer, alkyd,

chlorinated rubber, cellulose lacquer, etc.;

(c) coatings formed by dipping in a hot melt of plastic such as

polyethylene, acrylic, and vinyl;

(f) coatings formed by using a powdered plastic and melting the

powder to form a coating using many different TPs

There are cold curing coatings and baldng coatings that principally use

TS plastics They include polyurethane, epoxy, polyester, alkyd, acrylic, phenolic, and urea-formaldehyde Curing occurs in which drying is by a chemical reaction between the molecules of the binder (Chapter 1) If the reaction occurs at room temperature the products are described as cold curing coatings If temperatures of 70C (158F) or higher are necessary to cause rapid reaction, the materials are known as baldng coatings In view of the many different ldnds of chemical reactions that are now used to produce insoluble coatings, the term convertible coating is used

There are the popular paints containing water They are called water- base, water-thinned, aqueous, etc These water-based paints include

latex or emulsion paints made with plastics (acrylic, polyvinyl acetate, etc.) Over a century ago the original water-base paints used casein and the emulsion oil paints containing alkyd resin and water Latex paints using butadiene-styrene developed during the 1940s They were referred to as rubber base paints that lacked ruggedness During the 1950s the acrylic emulsion type paint was introduced for interior and exterior use These more expensive latex-plastic coatings continue to be very popular since they eliminate solvent fumes, reduce fire and explosion hazards, improve worldng conditions, and reduce fire insurance rates

These TP based coatings have a low solids content because their relatively high MWs require large amounts of solvent to reduce the viscosity to levels low enough for application The increasing costs of solvents and air pollution regulations limiting the emission of volatile organic compounds (VOCs) have led to the increasing replacement of these coatings with lower-solvent or solventless coatings However large-scale solvent-coating production systems continue to be economically beneficial when used with available solvent recovery systems

Paints containing water (latexes) have a dispersion of high-MW plastic

in water This condition results in the desirable low solvent emission Because the TP is not in solution, the rate of water loss is almost independent of composition until it is close to complete evaporation