Coating Methods, Powder Technology.pdf

570

COATING METHODS, POWDER TECHNOLOGY

Vol. 5

COATING METHODS,

POWDER TECHNOLOGY

Introduction

Powder coating is a process for applying coatings on a substrate using heat fusible

powders. Materials used in the process are referred to as coating powders, ﬁnely

divided particles of organic polymer, either thermoplastic or thermosetting, which

usually contain pigments, ﬁllers, and other additives. After application to the

substrate, the individual powder particles are melted in an oven and coalesce

to form a continuous ﬁlm having decorative and protective properties associated

with conventional organic coatings.

The origin of powder coating technology dates back to the late 1940s when

powdered thermoplastic resins were applied as coatings to metal and other sub-

strates by ﬂame spraying. In this process, a powdered plastic was fed through a

ﬂame spraying apparatus where the plastic particles are melted and propelled by

the hot gases to the substrate. A patent issued in Great Britain to Schori Met-

allising Process, Ltd., in 1950 described a process for forming a coating in which

powdered thermoplastics were applied to a heated substrate by dipping or rolling

the heated article in the plastic powder (1). This process was difﬁcult to practice,

however, and never achieved commercial success.

A major breakthrough in powder coating occurred in the mid-1950s, when

Erwin Gemmer conceived the ﬂuidized-bed coating process, in which a heated

object is dipped into a ﬂuidized bed of powder. Gemmer was involved in developing

ﬂame spraying processes and materials in the laboratories of Knapsack-Griesheim

(Hoechst), a manufacturer of specialty gases, and was searching for a more efﬁcient

method than ﬂame spraying for coating objects with powder. The ﬁrst patent

applications were ﬁled in Germany in May 1953, and the basic patent was issued

Vol. 5

COATING METHODS, POWDER TECHNOLOGY

571

in September 1955 (2). The ﬁrst United States patent was issued in 1958 (3),

and the Polymer Corp., Reading, Pa, acquired rights to the Knapsack-Griesheim

patents. The Polymer Corp. mounted an aggressive effort to develop, license, and

sell ﬂuidized-bed coating technology in North America. However, acceptance of

this coating process was rather slow. In 1960, the annual sales of coating powders

in the United States were below 450 t, in part because of a lack of expertise in the

methodology. In addition, the available powder coating materials were expensive,

efﬁcient production techniques had not been worked out, and volume of production

was low.

Today, powder coating is widely accepted, with thousands of installations in

the factories of original equipment manufacturers (OEMS) and custom coating job

shops. It is the preferred method for coating many familiar items such as lawn and

garden equipment, patio and other metal furniture, electrical cabinets, lighting,

shelving and store ﬁxtures, and many automotive components.

In the ﬂuidized-bed coating process, the coating powder is placed in a con-

tainer having a porous plate as its base. Air is passed through the plate causing

the powder to expand in volume and ﬂuidize. In this state, the powder possesses

some of the characteristics of a ﬂuid. The part to be coated, which is usually metal-

lic, is heated in an oven to a temperature above the melting point of the powder

and dipped into the ﬂuidized bed where the particles melt on the surface of the

hot metal to form a continuous ﬁlm or coating. Using this process, it is possible to

apply coatings ranging in thickness from about 250 to 2500

m (10–100 mils). It is

m, and therefore, ﬂuidized-bed

applied coatings are generally referred to as thick-ﬁlm coatings, differentiating

them from most conventional paint-like thin-ﬁlm coatings applied from solution

or as a powder at thicknesses of 20–100

m (0.8–4 mils).

In the electrostatic spray process, the coating powder is dispersed in an air

stream and passed through a corona discharge ﬁeld where the particles acquire an

electrostatic charge. The charged particles are attracted to and deposited on the

grounded object to be coated. The object, usually metallic and at room tempera-

ture, is then placed in an oven where the powder melts and forms a coating. Using

this process it is possible to apply thin-ﬁlm coatings comparable in thickness to

conventional paint coatings, ie, 20–75

m. A hybrid process based on a combina-

tion of high voltage electrostatic charging and ﬂuidized-bed application techniques

(electrostatic ﬂuidized bed) has evolved, as well as triboelectric spray application

methods. Powder coating methods are considered to be fusion-coating processes;

that is, at some time in the coating process the powder particles must be fused or

melted. Although this is usually carried out in a convection oven, infrared, resis-

tance, and induction heating methods also have been used. Therefore, with minor

exceptions, powder coatings are factory applied in ﬁxed installations, essentially

excluding their use in maintenance applications. Additionally the substrate must

be able to withstand the temperatures required for melting and curing the poly-

meric powder, limiting powder coating methods to metal, ceramic, and glass (qv)

substrates for the most part, although recently some plastics and wood products

have been powder coated successfully.

Compared to other coating methods, powder technology offers a number of

signiﬁcant advantages. These coatings are essentially 100% nonvolatile, ie, no sol-

vents or other pollutants are given off during application or curing. They are ready

difﬁcult to obtain coatings thinner than about 250

572

COATING METHODS, POWDER TECHNOLOGY

Vol. 5

to use, ie, no thinning or dilution is required. Additionally, they are easily applied

by unskilled operators and automatic systems because they do not run, drip, or

sag, as do liquid (paint) coatings. The rejection rate is low and the ﬁnish tougher

and more abrasion resistant than that of most conventional paints. Thicker ﬁlms

provide electrical insulation, corrosion protection, and other functional properties.

Powder coatings cover sharp edges for better corrosion protection. The coating ma-

terial is well utilized: overspray can be collected and reapplied. No solvent storage,

solvents dry off oven, or mixing room are required. Air from spray booths is ﬁltered

and returned to the room rather than exhausted to the outside. Moreover, less air

from the baking oven is exhausted to the outside thus saving energy. Finally, there

is no signiﬁcant disposal problem because there is no sludge from the spray booth

wash system. Any powder that cannot be reclaimed and must be discarded is not

considered a hazardous waste under most environmental regulations. Although

the terms coating powder and powder coating are sometimes used interchange-

ably, herein the term coating powder refers to the coating composition and powder

coating to the process and the applied ﬁlm.

Coating powders are frequently separated into decorative and functional

grades. Decorative grades are generally ﬁner in particle size and color and ap-

pearance are important. They are applied to a cold substrate using electrostatic

techniques at a relatively low ﬁlm thickness, eg, 20–75

m. Functional grades

m, using ﬂuidized-bed, ﬂocking,

or electrostatic spray coating techniques to preheated parts. Corrosion resistance

and electrical, mechanical, and other functional properties are more important in

functional coatings.

Coating powders are based on both thermoplastic and thermosetting resins.

For use as a powder coating, a resin should possess low melt viscosity, which affords

a smooth continuous ﬁlm; good adhesion to the substrate; good physical proper-

ties when properly cured, eg, high toughness and impact resistance; light color,

which permits pigmentation in white and pastel shades; good heat and chemical

resistance; and good weathering characteristics, ie, resistance to degradation by

uv light, hydrolysis, and environmental pollutants. The coating powder should

remain stable on storage at 25 ◦ C for at least 6 months and should possess a sufﬁ-

ciently high glass-transition temperature T g so as to resist sintering on storage.

The volume of thermosetting powders sold exceeds that of thermoplastics by

a wide margin. Thermoplastic resins are almost synonymous with ﬂuidized-bed

applied thick-ﬁlm functional coatings and ﬁnd use in coating wire, fencing, and

corrosion resistant applications whereas thermosetting powders are used almost

exclusively in electrostatic spray processes and applied as thin-ﬁlm decorative

and corrosion resistant coatings.

Thermoplastic resins have a melt viscosity range that is several orders of

magnitude higher than that of thermosetting resins at normal baking tempera-

tures (see Table 1). It is, therefore, difﬁcult to pigment thermoplastic resins suf-

ﬁciently to obtain complete hiding in thin ﬁlms, yet have sufﬁcient ﬂow to give a

smooth coating since incorporation of pigments reduces melt ﬂow even further. In

addition, thermoplastic resins are much more difﬁcult to grind to a ﬁne particle

size than thermosetting resins, and so grinding must usually be carried out under

cryogenic conditions. Because powders designed for electrostatic spraying gener-

ally have a maximum particle size of about 75

m (200 mesh), the thermoplastic

are usually applied in thick ﬁlms, eg, 250–2500

Vol. 5

COATING METHODS, POWDER TECHNOLOGY

573

Table 1. Physical and Coating Properties of Thermoplastic Powders a,b

Property

Vinyls

Polyamides Polyethylene Polypropylene

PVDF c

Melting point, ◦ C

130–150

186

120–130

165–170

170

Preheat/postheat

temperatures, ◦ C d

290–230

310–250

230–200

250–220

230–250

Speciﬁc gravity

1.20–1.35 1.01–1.15

0.91–1.00

0.90–1.02

1.75–1.90

Adhesion e

G–E

Surface

appearance f

Smooth

sl OP

Gloss, Gardner

60 ◦ meter

40–90

20–95

60–80

Hardness, Shore D

Resistance e , g

30–55

70–80

30–50

40–60

70–80

Impact E E G–E G G

Salt spray G E F–G G G

Weathering G G P P E

Humidity E E G E G

Acid h E F E E E

Alkali h E E E E G

Solvent h F E G E G–E

a All powders require a primer and pass the ﬂexibility test, which means no cracking under a 3-mm

dia mandrel bend.

b From Encyclopedia of Chemical Technology , 4th ed.

c Poly(vinylidene ﬂuoride).

d Typical ranges.

e E = excellent; G = good.

f OP = orange-peel effect; sl OP = slight orange-peel effect.

g F = fair; P = poor.

h Inorganic; dilute.

powders are predominant in the ﬂuidized-bed coating process where heavier coat-

ings are applied and a larger particle size can be tolerated. Fluidized-bed powders

typically contain only about 10–15% of particles below 44

m (70 mesh).

Most thermoplastic coating powders require a primer to obtain good adhesion

and priming is a separate operation that requires time, labor, and equipment and

typically involves solvents. In automotive applications, some parts are primed

by electrocoating. Primers are not usually required for thermosetting powder

coatings.

Thermoplastic Coating Powders

Thermoplastic resins used in coating powders must melt and ﬂow at the appli-

cation temperatures without signiﬁcant degradation (see Table 1). Attempts to

improve the melt ﬂow characteristics of a polymer by lowering the molecular

weight and plasticizing or blending with a compatible resin of lower molecular

weight can result in poor physical properties or a soft ﬁlm in the applied coat-

ing. Attempts to improve the melt ﬂow by increasing the application temperature

m (325 mesh), whereas

the high end of the particle-size distribution ranges up to about 200

574

COATING METHODS, POWDER TECHNOLOGY

Vol. 5

5 min. The principal

polymer types are based on plasticized poly(vinyl chloride) [9002-86-27] (PVC),

Polyamides, plastics (qv), and other specialty thermoplastics. Thermoplastic coat-

ing powders have one advantage over thermosetting coating powders: they do not

require a cure and the only heating necessary is that required to complete melting

or fusion of the powder particles. Thermoplastic resins have uses in coating wire,

fencing, and other applications where the process involves continuous coating at

high line speeds. Typical properties of thermoplastic coating powders are given in

Table 1.

PVC Coatings. All PVC powder coatings are plasticized formulations (see

V INYL C HLORIDE P OLYMERS ). Without plasticizers (qv), PVC resin is too high in

melt viscosity and does not ﬂow sufﬁciently under the inﬂuence of heat to form a

continuous ﬁlm. Suspension and bulk polymerized PVC homopolymer resins are

used almost exclusively because vinyl chloride–vinyl acetate and other copolymer

resins have insufﬁcient heat stability. A typical melt-mixed PVC coating pow-

der formulation is given in Reference 4. Dispersion grade PVC resin is added

in a postblending operation to improve ﬂuidizing characteristics (5). Additional

information on the formulation and application of PVC coating powders can be

found in Reference 6. While most PVC coating powders are made by the dry-blend

process, melt-mixed formulations are used where superior performance, such

as in outdoor weathering applications and electrical insulation, is required (see

Fig. 1). Almost all PVC powder coatings are applied by the ﬂuidized-bed coating

process. Although some electrostatic spray-grade formulations are available, they

are very erratic in their application characteristics. The resistivity of plasticized

PVC powders is low compared to other powder coating materials and the applied

powder quickly loses its electrostatic charge. Dishwasher baskets are coated with

ﬂuidized-bed PVC powder. Other applications are various types of wire mesh and

chain-link fencing. PVC coatings have a very good cost/performance balance that

is difﬁcult to match with any of the other thermoplastic materials. Properly for-

mulated PVC powders have good outdoor weathering resistance and are used in

many applications where good corrosion resistance is required. These coatings are

also resistant to attack by most dilute chemicals except solvents. In addition, PVC

coatings possess excellent edge coverage.

Powder coatings as a class are superior to liquid coatings in their ability to

coat sharp edges and isolate the substrate from contact with corrosive environ-

ments. PVC coatings are softer and more ﬂexible than any of the other powder

coating materials. Primers used for PVC plastisols have been found generally

suitable for powder coatings as well (7).

Polyamides. Coating powders based on polyamide resins have been used

in fusion-coating processes from the earliest days. Nylon-11 [25587-80-9] has been

used almost exclusively; however, coating powders based on nylon-12 [24937-16-4]

also have been sold. The properties of these two resins are quite similar. Nylon-6

[25038-54-4] and nylon-6,6 [32131-17-2] are not used because the melt viscosities

are too high.

≤

are limited by the heat stability of the polymer. If the application temperature is

too high, the coating shows a signiﬁcant color change or evidence of heat degra-

dation. Most thermoplastic powder coatings are applied between 200 and 300 ◦ C,

well above the generally considered upper temperature limits for adequate heat

stability. However, the application time is short, usually

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: