Coatings of Polymers and Plastics part 15 doc

TABLE 8 Application Parameters for Waterbased BasecoatsFirst spraying operation Metallic basecoats approximately 6–8µm dry film thicknessAir speed 0.4 to 0.6 m/sRelative air humidity 60+

TABLE 8 Application Parameters for Waterbased Basecoats

First spraying operation Metallic basecoats approximately 6–8µm dry film

thicknessAir speed 0.4 to 0.6 m/sRelative air humidity 60+/− 5%

Room temperature 23+/− 3°CIntermediate flash-off Approximately 2 min @ 23+/− 3°C

Second spraying operation Metallic approximately 6–8µm dry film thickness

Air speed 0.5 m/s+/− 0.1 m/sRelative air humidity 60+/− 5%

Room temperature 23+/− 3°CAir lock (Approximately 3 to 5 min.)

Final flash-off Approximately 6 min at 60°C (drier rated for 80°C)

on work piece 2+/− 0.5 m/sCooling The substrate in the discharge lock of the blowing

tunnel with fresh air to 30°C

for the processing of waterborne paints A case-to-case examination has to bemade as to whether integration into existing painting lines is possible However,the paint supply system and the application technology must always be adapted.The most widely used pneumatic application process, manual and/or auto-matic processing, is described in detail as an example In general, the parametersalso apply to electrostatic processing Further details are given in the next para-graph

The effect of the humidity (see processing range) is more crucial withwaterborne basecoats than with conventional systems Even after a 10-minuteflash-off period there is still too much residual solvent in the paint film under

TABLE 9 Pneumatic Spray Application Parameters

Hand spray gun, stainless steel DeVilbiss JGV-563 St

Automatic Gun, alternatively DeVilbiss AGMD, ABB, Behr

Nozzle (∅ mm) 1.1/1.4

Air cap (DeVilbiss) 765/789/797

Sprayer air pressure (bar) dynamic 4–5 5–6

Material flow rate (ml/min.) 200–400 200–400

Object distance (mm) 250–300 250–300

Booth temperature (°C) 23+/− 3 23+/− 3

Relative air humidity (%) 60+/− 5 60+/− 5

(best sprayingconditions)Down draft (m/s) 0.5+/− 0.1 0.5+/− 0.1

normal booth conditions and therefore a special evaporation tunnel is needed.

In the actual painting of plastics a combination of infrared drying and quent blowing off with heated air has not proved successful for the forced evap-oration of waterborne basecoats and instead pure convection drying is recom-mendable Figure 9 shows a temperature curve for this basecoat clearcoatapplication and bake process

subse-Over the last years, several new concepts for the integration of waterbornebasecoats into existing paint shops have been introduced Due to the three-dimensional, complicated geometry and poor temperature stability of many plas-tic parts, development of high-temperature radiation dryers (like infrared [IR]and ultraviolet [UV]) could not be introduced With the exception of some mid-dle- or long-wave infrared heaters, and also a few catalytic gas dryers (forsmaller and more flat parts) in Great Britain and in France, process engineersand plastic coaters studied alternate dryers that operated at low temperature andworked on the principle of condensation For these types of systems, closed aircirculation is needed and the relative humidity must be less than five gramsper kilogram of air Processing temperatures are approximately 40°C Theseconditions offer the benefit of a shorter process and the typical cooling timeperiod that is required after drying is not necessary

F IG 9 Temperature curve for waterbased basecoat–waterbased clearcoat

In view of the increasing significance of painting plastic surfaces statically, this process is explained in more detail Electrostatic painting hasbeen the main standard method for coating metallic substrates for many decades.The advantage of this technique is the low loss of paint mist compared withpurely pneumatic spraying processes As a result, it is a cost-effective method

electro-of delivering paint to a part

Particularly more and more color harmony is required between the carbody and the plastic parts This can be very challenging because most off-lineapplication methods are not at all similar to that of the car-body paint lines In

an attempt to improve this color harmony, conveyor systems have been built tohang and carry the plastic parts in car-body position through the plastic paintline Also the application concept with the typical electrostatic atomizer is inuse, similar to that used with the car body Examples of robot types used inEurope are ABB and Fanuc

Electrostatic units operate on two principles, the purely electrostaticmethod and those methods with additional auxiliary energy for paint atomiza-tion One thing that all processes have in common, however, is the fact thatelectrostatically charged paint droplets when sprayed are transported to thegrounded part under the action of the electrostatic field and deposited on thejob Electrostatic application on plastics is a little more sensitive than the pro-cess used on the car body, as the typical plastic part is not inherently conductive.For optimum transfer efficiency the experience is, that a specific system resis-tance has to be less than one Mega-Ohm Best results are measured in a combi-nation of waterborne basecoat with an underlying layer of conductive primer.The high conductivity of the wet waterborne paint increases the transfer effi-ciency higher than normal

Electrostatic painting using high-rotation atomization is characterized by

a considerably higher transfer efficiency (24,25) compared with pneumatic omization Spraying is performed purely mechanically at the bell edge Thisprocess promises to exhibit the highest efficiency for coating plastic parts Un-der the pilot plant conditions at DuPont, paint utilization of up to 60 percentwas attained on bumpers whereas in a conventional pneumatic plant approxi-mately only 30 percent is achieved In practice, these figures are considerablylower when applied through electrostatic application owing to poor grounding

at-In spite of these advantages, the use of electrostatic high-rotation sprayers hasfor a while only been partially implemented for the application of “effect” base-coats Normally, variations in color and a different flop behavior compared withpneumatic atomization occurs, which would clearly become noticeable as anoptical flaw in the repair of parts without electrostatics in series production or

in the field

The color and effect deviations that are usually seen are attributed to thedifferent atomization and transport conditions of the paint droplets, the different

evaporation behavior of the solvents contained in the atomized droplets and thedifferent kinetics of the droplets when they impact on the surface being painted.These differences induce an alignment of the aluminum platelets or other effectpigments causing the color deviation (see Fig 10) However, in spite of thislimitation, currently between 50 and 70 percent of the dry film thickness ofeffect basecoats can be applied electrostatically without any loss of optical qual-ity To apply as much as possible, ideally up to 100 percent of the basecoat,through electrostatical application requires close cooperation of all partnersthroughout the whole development process, starting with the OEM stylists.Surface tension, viscosity, and the paint thickness influence the dropletsize distribution and the average droplet diameter just as much as the angularvelocity, diameter, and specific design details of the sprayer in conjunction withthe paint throughput In this context, the very low electrical resistance of thewater in water-thinnable basecoats deserves special attention If the paint issupplied from a closed-circuit pipe, the high voltage present at the sprayinghead owing to the paint column created can be discharged into the entire supplysystem The possibility of completely interrupting the paint column with opti-mum safety by means of intermediate replenishing tanks operated in isolationwas previously used with waterborne fillers in automobile painting Figure 11shows a typical replenishing tank system As a replenishing tank located in thevoltage cascade and requiring a relatively large space was needed for each color,

F 10 Aluminum- or mica-flake orientation in HR-bell and pneumatic spray

F IG 11 Intermediate replenishing tank system, ohmic insulated for electrostaticwaterbased basecoat application (Courtesy of: LACTEC GmbH, Rodgau, Germany.)

it was not possible to transfer such a solution to basecoats with their widevariety of shades.

At present, the standard solution for basecoats is therefore still the concept

of high-rotation atomization with external charging In this case, the paint ismerely sprayed by high rotation The electrostatic charge is created in a secondstage by the air ions attaching themselves to the paint droplets in the high-voltage field between the external charging electrodes and the object beingpainted The field geometry, voltage, shaping air, air velocity, and air humidity

in the booth must be set in relation to each other so that return-spray effects,which could lead to contamination of the electrodes, are avoided

7.3 Clearcoats

Due to the plastics and molding conditions used by the European automotiveindustry, the maximum curing condition for bumpers, grilles, side claddingsand most other plastic items is 90°C (194°F) Therefore, since the early 1980sisocyanate curing 2K clearcoats have been used Initially, highly flexible, poly-ester resins were needed in the backbone to avoid deterioration of the low-temperature impact resistance of the painted part A cryogenic polishing tech-nique, using liquid nitrogen, was carried out to touch up any of these parts whenand if defects were seen More recently, clearcoats have been developed withbuilt-in flexibility providing good low-temperature impact, which can be pol-ished at ambient temperature

In terms of resin chemistry, the clearcoats are based on a tional polyester and acrylic-resin blend The polyester is responsible for provid-ing the high flexibility at low temperature The hardener is based on hexamethy-lene diisocyanate (HDI) trimer Ultraviolet absorbers (UVA), and hinderedamine light stabilizers (HALS) are additives added to absorb UV light, protectthe basecoat pigments and to quench free radicals that could deteriorate anddecompose the backbone resins

hydroxy-func-To meet the need for very robust products, new products are continuallybeing developed that may be applied at low cost through high pressure sprayguns or through high-efficiency bells with little risk of popping, nonuniformfilm build, or even sagging over a wide range of film builds High skills areneeded in the formulation chemist Formulation tools are needed to build struc-tural viscosity to avoid sagging “in the booth,” thixotropy is needed to avoidsagging in the “flash off” zone, and temperature-induced viscosity is needed tocontrol the film in the oven on vertical areas of the molded part

The typical product used on the European continent is a medium solids2K clearcoat In the United Kingdom however, local authorities require productswith VOCs less than 420g/l Clearly, these higher solid products have limiteduse compared to their medium solid counterparts when highly effective applica-

tion with smooth orange peel, even at low film build, is needed In addition tosolvent-based clearcoats, water-based clearcoats have also been developed andthese trial products are under evaluation for industrial use (26) Figure 12 shows

a typical application of a waterbased clearcoat

To a growing extent, the European automotive industry uses plastic and setting materials for body applications Two materials, SMC and PPO/PA aredominating in this area It was in the early 1980s that SMC first appeared onmiddle volume vehicles and since then it has been used to a higher or lowerextent, depending on the automotive OEM Typical application for these materi-als are tailgates, an early example being the Audi Avant Today, SMC plays anincreasing role in this area and a number of variations of this technology arecurrently used

thermo-A SMC trunk lid is assembled on DaimlerChrysler’s S-Klasse Coupe Thispart is precoated with an in-mold coating (IMC) and a black conductive primer

It is assembled to the body and passed through a cathodic electrodepositiontank, with no coating adhering to the SMC part The black conductive primerallows for electrostatic application of primer surfacer and topcoat along with theautomobile body Renault (VelSatis) and Volvo (V70 station wagon) tailgatesare coated completely off-line, without the use of IMC, and are coated with adual primer system consisting of highly conductive primer and a light gray,

F IG 12 Application of waterbased clearcoat A few minutes after application, thewet film changes from “milky” to “transparent”

nonconductive primer on top Renault also topcoats SMC fenders that areprimed with a conductive powder coating.

Despite many improvements in SMC technology, the major problem ofpopping has not been completely eliminated Porosity, due to gas inclusionsmainly on the edges and wherever the part has been stressed, limits the scope

of many SMC applications To cope with this inherent porosity best, two ries” are followed The first is to have a process starting with high temperatures

“theo-of the primer bake In this early stage, any pores popping out can be filled withputty Due to the lower temperatures used later in topcoat bake, a minimum ofpores are assumed to pop due to a sealing effect of the paint film The primersused in this method are 140°C melamine-cured systems More recently, accord-ing to the second theory, the overall process to coat SMC should be performed

at low temperature Low-bake 2K primers have been commercialized to tively reduce the amount of visible porosity

effec-In increasing volume, hard polyurethane (PU) composites are commonlyused for body applications, like hardtops Typically, a reinforced molding cov-ered by a 3 mm skin, is coated off-line In addition to thermosetting materials,highly temperature-resistant thermoplastic polymers have been introduced tothis body technology A particularly suitable thermoplastic material for bodycomponents is the blend of PPO/PA The structure is a matrix with sphericaldomains due to the noncompatibility of the polymers As the PA phase is semi-crystalline, shrinkage phenomena of the plastic also have to be considered

bake to preshrink the part for optimal body fit when assembled to the vehicle

A few examples illustrate as with SMC parts, for PPO/PA body nents, many different coating options are used and no general coating processhas been established DaimlerChrysler’s A-Classe has a Noryl tailgate that iscompletely coated off-line in body color The fenders of the same car however,are primed using a conductive primer and are assembled “after e-coat” for on-line coating This is accomplished using a functional-surfacer, water-based base-coat and powder slurry clearcoat Other fenders like that on the Audi A2, arecoated completely off-line For Renault’s Scenic and Clio models, black conduc-tive PPO/PA is used so that the part can be coated on-line after passing throughthe electrocoat bath

compo-9 INTERIOR AUTOMOTIVE COATINGS

Interior design of automotive vehicles in Europe today is an important factor intheir sales success The design stylists try to create a complete harmony of theinterior trim using a combination of leather, woven fabrics, wood, and plastic

surfaces In our perception, plastics don’t meet the stylist’s wish for idle orluxurious materials and hence the trend is to apply a coating finish to make theplastic surface more appealing.

A wide range of finishes is at disposition of the designer Traditional

“low-gloss coatings” can make plastic components made of different materials

or molded by different techniques, look alike and offer a elegant finish cially when in dark colors, they eliminate or minimize reflection in the underwindow area of the car’s interior Lighter colors are the trend for componentsbelow this area where low-gloss finishes can look comparable to natural materi-als Additional highlight styling elements are high-gloss metallic or pearlescentcoated trim moldings Galvanic metallizing of ABS-plastics in chrome, brushedaluminium, or other special effects are used and usually coated with a specialclearcoat to protect the finish Alternatively, chrome effect basecoats are avail-

graphic and colored pattern or picture have been developed and special clearcoatsare also needed to support the appearance of the finish with respect to gloss,smoothness, and light refraction Functional coatings combine both an attractivedesign with functional needs, for example laserable coatings to label buttons, dis-plays, or special paints to provide physical properties to material surfaces.The automotive stylist has to focus on more than just what is seen withthe human eye, he has to appeal to the other senses Antisqueak coatings can beapplied to avoid unwanted noise that can be created when adjacent plastic sur-faces rub against one another Interior coatings can add an idle smell to thecomponent and also give the perception of the surfaces when touched by thehand Based on the human experience with natural matters, a range of highelasticity materials with certain friction to our fingertips is pleasantly perceivedand coatings providing this effect are called softcoatings Very often, manualoperation elements like gear knobs, door handles, hand brakes, and radio knobsare given soft coatings More and more all surfaces within the reach of thedriver and passenger now have this soft finish including middle consoles, dash-board inlets, armrests, and airbag covers

According to simple model considerations, soft coatings can be accuratelydescribed in terms of their elasticity modulus and frictional resistance Whenmoving our fingertips along the surface of a soft coating, a minimal shift of thefilm surface versus the lower face bonded to the substrate occurs and can be

“felt.” What we feel in physical terms is the sheer modulus of the film Sheermodulus is related to tensile modulus and, using Poisson’s constant (P), can betransformed into the other using the following equation:

Hence, dynamic tensile measurements are suitable to characterize paintmaterials in terms of a number of factors such as a “storage factor” and a “loss

factor.” Both of these factors describe the energy of distortion that can be ered or lost by heat formation when a paint film is sheared Other available datathat may be obtained includes “time lag” of the periodically applied shear stressand shear strain Both these are temperature influenced and can be seen in Fig-ure 13 According to the shear model, thicker films exhibit better soft effectsbecause a defined shear stress gives more shear strain Typical soft coatings areapplied in the range of 40 to 60µm.

recov-Additives and pigments can influence the type of soft effect we feel out dramatically changing the sheer modulus and the friction of our fingers tothe paint film is responsible for this When testing textured surfaces rather thansmooth ones, things can get quite complicated and on these surfaces, qualifica-tion by trained test personnel is the only way to consistently characterize softcoats

with-In a somewhat simplified characterization, different types of soft-touchcoatings can be represented in terms of resin shear modulus and frictional be-havior of the film surface (see Fig 14) For better quantification of the latter,

“artificial” fingertips for test purposes are currently under evaluation

For soft finishes, solventbased and waterbased coatings are available both

in middle and northern Europe, with the waterbased widely dominating the ket The chemical basis of waterbased finishes is based on a 2K polyol-isocya-nate Suitable polyols are special aqueous polyurethane dispersions The isocya-nate is provided in liquid form and usually contains a small amount of suitablesolvents Mixing to get the paint ready to spray can be done in small lots asmost of these 2K materials exhibit a pot life of about one hour at ambienttemperature However, it is best to apply soft coatings through 2K automatedmix equipment with modified statical mixers

mar-These waterbased soft paints adhere to many plastics Clearly ABS nates in the interior trim market In cases where polypropylene blends are used,

domi-a consistent pretredomi-atment by fldomi-aming or fluorindomi-ation is necessdomi-ary to get domi-adequdomi-ateadhesion For the application of waterbased soft paints, some pertinent data isgiven in Table 10

Soft coatings mostly are solid colors like black, gray, beige, blue, andothers In addition to this, certain metallic effects such as pearlescent effecttextures can be provided Also, soft clearcoats over special metallic basecoatsalso have become available recently

In addition to standard applications, special soft coatings exist that offer

a wide range of options For example, infrared reflecting coats have been ented (27), and are available for dashboards to help reduce component tempera-

can be reduced, the dashboards can be constructed much easier, cheaply, and atless weight Antibacterial soft coatings have recently been offered for door han-dles, steering wheels, and gearknobs of rental cars In addition, flame-retardant