> 1 1

\

APPLIED 2/ POLYMER SYSTEMS, INC. 4302 East loth Avenue, #404, Tampa, Florida 33605 (813) 247-3065 FAX (81 3) 247-5923

NOVEL PLASMA SPRAY COATINGS PROCESS FOR

THERMOPLASTICS AND HIGHLY FILLED SYSTEMS

Gary K. Sweet

and

William W. Bristowe

of

Applied Polymer Systems, Inc.

. .-

NOVEL PLASMA SPRAY COATINGS PROCESS FOR

THERMOPLASTICS AND HIGHLY FILLED SYSTEMS

INTRODUCTION

A new cost effective plasma spray process and specially formulated materials for use with the process have been developed. These materials consist of a wide variety of products based mostly on thermoplastic polymers with the option of adding fiiers or fibrous reinforcement. Such thermoplastics include nylons, fluoropolymers and copolymers, polypropylene, and polyethylene, including ultra-high molecular weight, high density polyethylene (UHMWPE), to name a few. Fillers can be included to enhance abrasion resistance, fire retardancy, electromagnetic radiation absorption, as well as slip properties. These materials can be sprayed onto vessels, structures, and various parts for in-house use or on-site treatment. Previously these materials have been limited to the following processing methods: electrostatic spraying followed by an oven baking operation, a flame spraying, or hot dipping, followed by an oven fusing process. This newly developed plasma spray technology now permits substrates, especially aerospace plastic materials, to be field sprayed with nominal heat input, producing high bond strength and high density coatings.

APPLIED POLYMER SYSTEMS, INC. has eliminated the disadvantages associated with other spray processes. This newly patented system provides the ability to field spray any size structure with coatings as thin as 2 mils or as thick as 2.00 inch. Shielding, heat deflection, erosion, corrosion, acoustical, infrared, and antifouling protection result from the application of the plasma spray system.

PRINCIPLES OF THE PROCESS

The term "plasma" is often considered in the scientific community as the fourth state of matter after solid, liquid, and gas. This extremely hot substance consists of free electrons and positive ions. Although it conducts electricity, it is electrically neutral.

The system utilizes argon gas passing through an electric arc between an anode and a cathode. The carrier gas loses one of its electrons and becomes a highly energetic, extremely hot plasma. As the plasma leaves the internally water-cooled plasma generator in the gun, powdered thermoplastic formulations and inert gas are introduced into the stream in a precisely controlled manner (see Figure 1). As the material is caught up in the high velocity hot plasma stream, it becomes molten and is projected against the surface being coated at subsonic (or, in some applications, supersonic) speeds. When individual particles impact against the surface at high speeds, thermal and mechanical energies are transferred to the substrate, producing forces which favor high level bonding.

ADVANTAGES OF THE PLASMA SPRAY PROCESS

a Elimination of Preheating, - The part to be coated does not usually need to be preheated. Sufficient thermal and kinetic energy are present in the thermoplastic powder to allow flow into the substrate crevices.

a High deposition spraying rates - Manual spraying at up to 15 lbs per hour and robotic operations at up to 30 lbs per hour. (Two spray guns.)

e Multilayered Coatings - All thermoplastic powder particles become molten within the shielded plasma column and rapidly fuse on contact at the high velocity to either the substrate or the previously deposited layer. There is no need to interrupt the powder feeding operation when spraying a part in order to reheat the coating previously applied, as the high thermal energy introduced into the new particles causes the previously deposited layer of coating to become instantaneously molten and allows for coating integrity.

a Inert Atmosphere - Argon, an inert gas, provides pressure for powder feeding and is the primary gas. Secondary gases may be selected from nitrogen, hydrogen, or helium. These again are basically inert. This insures a non-reactive environment during the flight of the molten thermoplastic particles. It also ensures that the deposited thermoplastic, as well as the treated surface, is pure and free of oxides, moist!;rre, and c o n t a ~ i n m s dlmhg the c m h g prmess.

a Minimal Surface PreDaration - Preparation of metal substrate requires grit blasting to roughen and remove oxides from the surfaces. Plastic or composite substrates only require solvent wiping of the surface to remove oils or waxes. Substrate temperatures should be about 50 degrees Fahrenheit or greater to reduce thermal shock.

0 Optional Robotic Application - Steps have been taken to simphfy the operation whereby highly skilled operators are not necessary. incorporated to place and move the gun, placing an even coating on the part.

A robot can easily be

Target Efficiencv-Spray Pattem and Distance - During the spray operation, the gun does not necessarily need to be held perpendicular to the surface being sprayed, as it does when flame spraying. Parts or structures can be sprayed up to 10 - 15 degrees off axis with good results. The spray pattern is oval and is dependent upon the spray distance from the gun to the work surface. The gun can be held as close as 2.0 inches away from the substrate, producing a pattern approximately 34 of an inch high. The maximum spray pattern is approximately 2.0 inches using a spray distance of 12.0 inches. Note: The powder injection point is outside the gun nozzle. Spray distance is not critical in order to obtain a good coating, however, special care should be taken when spraying the first coat. The first coat must be completely molten for maximum coating adhesion, and the distance it is held away should be from 2.0 - 5.0 inches. The remaining successive coats, which are necessary to obtain the required thickness, are then applied by oscillating back and forth relative to the part at a steady rate, as in spraying ordinary paints. Travel speed, feed rates, power settings, and the type of thermoplastic being sprayed should be taken into consideration. The gun should be moved at a fast enough rate so as not to overheat the surface (see Figure 2).

0 Controlled Heating and Melting - The Plasma Spray System developed by Applied Polymer Systems puts more material where you want it with a minimum of time and material expenditure. While the entire system is designed to achieve thermoplastic coating efficiency, the major contribution is made by the Applied Polymer Systems powder feeding system and gun. The Plasma system consists of the Plasma Spray Gun, Control System, Powder Feeder, Power Supply, Heat Exchanger, High Frequency Starter, and interconnecting hoses and cables. Many of these components, such as the Power Feeder, Control System, and Plasma Gun, are all especially designed for spraying thermoplastic type materials. Applied Polymer Systems has made major developments in gun design, powder injection, and powder feeding. The patented Gun is a co-axial flow venturi plasma generator with a water-cooled nozzle that surrounds the plasma column. This new design allows the same advantage of high velocity plasma spraying, while eliminating the high radiant heat and ultraviolet light that are associated with Plasma Flame Spraying. The system injects the powder into the interior of the co- axial flow nozzle. This tends to collimate the powder pattern, which improves the heat transfer to &e ~hemq~lastic pmiclees. “!Ie system dse hcorpmtes 5 circumferential vacuum design that is integrated into the gun which removes most vapors and smoke during spraying, and is filtered through a disposable filtering system.

THE THERMOPLASTIC PLASMA SPRAY UNIT

The Thermoplastic Plasma Spray Unit is self-contained, mounted on a portable skid, and consists of a closed-loop integrated system (see Figure 3) including:

Computer Controlled Remote Pendant - The computerized control system provides the ultimate in user friendliness. All of the systems on the skid are operated by the computer, which interfaces with the operator by means of a menu- driven, touch screen pendant. The optimum spray parameters for each polymer coating material are pre-programmed into the computer, and are then presented to the operator on the pendant screen. The operator then simply "points" to his selection, whereupon he is presented with a second screen display which will allow him to adjust certain parameters such as powder feed rate, arc current rate, etc. The touch screen flexibility and simplicity eliminates the user intimidation that is often experienced with other general purpose display and input devices. This pendant has a long cable, enabling the operator to make minor parameter changes at the gun, while spraying.

The computer uses STD-80 Series Bus Components and has been ruggedized for field applications.

High Volume Powder Feeders - A single high volume powder feed hopper is capable of delivering 15 lbs of lightweight plastic powder per hour up to 75 feet away for high production spraying.

Mass Flow Controllers - Mass flow controllers are used for maintaining tighter control over arc and powder feed gases. The controllers are monitored by the computer to maintain correct flow rates.

Thermoplastic Plasma Spray Gun - The patented gun is a co-axial flow, venturi plasma generator with a liquid-cooled nozzle that surrounds the plasma column. The new design allows the same advantages of high velocity plasma spraying, while eliminating the high radiant heat and ultra violet light that is associated with plasma flame spraying. This system does not inject powder into the interior of the plasma generator, but into the co-axial flow nozzle. This tends to collimate the powder pattern, which improves the heat transfer to thermoplastic particles, thereby improving deposit efficiency and coating quality.

iOii KW Power Suppiy - A io0 kw solid state DC power supply provides the Plasma Spray process with constant current.

Closed Loop Refrigerated Cooling System - A closed loop, refrigerated cooling system eliminates the need for an external water source during field spraying.

Vacuum Recovery System - A circumferential vacuum nozzle, integrated into the gun, removes overspray and vapors during the application and is filtered through a disposable cartridge.

(h) Electrical Distribution Equipment - A main breaker and branch breakers protect each individual component. The main breaker has a computer controlled remote trip capability for emergency stop.

(i) Interconnecting Hoses and Cables - The system is electrically wired to meet national electrical code. Gas hoses are supplied, including 75-foot liquid-cooled process cables to the plasma gun.

(i) Inert Gas Supply - The skid incorporates provisions for the installation of up to a 700 pound bottle of argon to provide gas for both the plasma arc and the powder feed.

MATERIALS FOR COATINGS

Materials used in APPLIED POLYMER SYSTEMS, INC.'s thermoplastic spray process consist of a wide variety of fmely ground materials such as nylons, polyolefins, polyesters, and fluorinated polymers. The list of polymers used to date are included in Table I. Specially developed formulas by Applied Polymers Systems, Inc. are BXTM and APSIFLONTM Series powders. They are available in a wide range of colors with pigments having high resistance to heat and exposure to sunlight.

In addition, extremely high levels of filler can be added to vary the properties of the thermoplastic. Exceptionally high loadings of filler can be obtained since the filler is added in neat form to the particulate thermoplastic. The actual wet mixing is encountered on the surface. Hence, the coating is formed insitu. This process allows higher loadings to be made in filler levels. Fillers can be added to achieve the properties mentioned in Table II.

(1) APSFIREB - Fire retardant coating. This is a polymeric substance which serves as a carrier for fire retardant and smoke suppressing chemicals. The polymer fuses immediately upon contact with the substrate and serves to exclude atmospheric oxygen, which reduces the amount of oxygen available to support combustion.

(2) POLYETHER BLOCK POLYAMIDE

This flexible nylon polymer is noted for the following properties:

MATERIAL PROPERTIES

good, flexible, rubber-like elasticity shore hardness from 73A to 70D temperature performance form -40°F to 180°F

COATING PROPERTIES

corrosion resistance resistance to abrasion and wear excellent impact resistance vibration and noise reduction electrical insulation anti-slip character

This material will find multiple new uses as a result of the development of Applied Polymer Systems, Inc.’s plasma spray process. Typical properties measured on such a coating were:

TYPE TEST STANDARD TEST VALUE

Adhesion ASTM D4541 2183 psi avg.

Impact Impact D2794 No fracture (20 idlb + - indirect)

Abrasion Resistance ASTM D 1242 90 mg

Shore D Hardness ASTM D 1484 63

The data as presented represents an ongoing test evaluation program and is subject to change as additional data is generated.

The substrate was 100 mils cold rolled steel, which was prepared by sand-blasting.

(3) NYLON 11

The versatility of Nylon 1 1 has been greatly enhanced by the development of the thermoplastic plasma spray system. Some important polymer properties that are inherent to Nylon 1 1 are:

resistant to chemicals including

resistant to petroleum products abrasion resistance properties excellent impact resistance,

low moisture uptake good stress cracking resistance good electrical insulation

pigmentable cosmetic-like surface in coatings

alkalies, certain acids, and solvents

especially to -94°F

The plasma spray processing of this material has led to many new applications, since neither pre-heating of the substrate nor its conductivity are criterion for applicability.

Typical properties measured on steel are reflected in the following data:

TYPE TEST STANDARD

Adhesion ASTM D4541 2143 psi avg.

Impact ASTM D2794 No Fracture (20 in/lb + - indirect)

Hardness Pencil Test 5H

Salt spray (5%) w

MEK (1,OOO RUB)

Unaffected

Unaffected

The substrate was 100 mils carbon steel which was surface treated prior to coating by sandblasting.

The data as presented represents an ongoing test evaluation program and is subject to change as additional data is generated.

(4) POLYVINYLIDENE FLUORIDE (PVDF)

This polymer is a fluorinated thermoplastic material which has been known for possessing the following unusual performance features.

MATERlAL PROPERTIES

high impact resistance and

abrasion resistance low coefficient of friction chemically resistant to acids, bases, halogens, gasoline

ozone resistant

non-wetting surface low flammability

9 pigmentable

toughness resists nuclear radiation

COATINGS

In coatings, this product has found markets in industries such as

chemical pharmaceutical manufacture paper and pulp aerospace pipe and valve manufacture marine pilings nuclear OEM coatings semi conductor food processing and handling

Applied Polymer Systems, Inc.’s new plasma spray process has added new vcrsacility tc! allow coatings to be made on non-conductive substrates and heat-sensitive substrates.

(5 ) ULTRA HIGH MOLECULAR WEIGHT POLYETHYLENE (UHMWPE)

Ultra-high molecular weight polyethylene is noted for the following properties:

MATERIAL PROPERTIES

outstanding chemical resistance lightweight to acids, bases, solvents, and excellent purity hydrocarbons FDA and USDA approved exceptional abrasion and wear physiological compatibility resistance low coefficient of fiiction good elastic behavior qualities low dielectric constant good temperature resistance (from -452°F to 194°F)

COATING PROPERTIES

This self-lubricating, non-stick, wear-resistant polymer can now be sprayed directly onto a substrate. Substrates can consist of composites or metals. Typical properties are:

TYPE TEST STANDARD TEST VALUE

Adhesion ASTM D4541 3642 psi avg.

Impact ASTM D2794 No Fracture (20 in/lb + - indirect)

Hardness Pencil Test 2H

Salt Spray (5%) - (1,000 hrs)

Unaffected

Water Absorption - Nil

The substrate was 100 mils cold rolled steel, which was prepared by sand- blasting.

These polymers are typical of polymers which have been used m-d is n o t to be restrictive.

APPLICATIONS

1. Abrasion Resistant Coating - Composites produced from high temperature resistant resins like polyamides, bismaleimides, or epoxies, are susceptible to abrasion. Particles such as sand or grit have a higher modulus than the resin rich surface from which these composites are made. When either of these abrasives strike and move across the surface, the higher-modulus material causes the lower-modulus material to wear.

Coatings can be developed with the plasma spray technology whereby high- modulus fillers can be incorporated into a thermoplastic matrix at extremely high filler loadings. Considering that the overall material property of the coating follows the rule of mixtures, the modulus of the coatings is the sum of the product of the modulus of filler, multiplied by the volume fraction, plus the modulus of the resin, multiplied by its volume fraction (see Equation 1).

M = V, M, + V, M,

At high filler loadings with a high modulus filler erosion by abrasion can be controlled.

Some fillers which have been used are tungsten carbidekobalt, alumina, and magnesium zirconate, to name just a few.

Applications for these coatings are for protection of composite helicopter blades from rain, sleet, or sand. Propeller blades on commercial aircraft are easily protected. Similarly, turbine blades on compressors can be coated and protected. Protection for holding devices where parts are fastened, commonly referred to as Shot Peening Tooling, for sandblasting are easily coated with an abrasion-resistant coating. Other examples are conveyor equipment for sand or gravel delivery and interiors of pipes where slurries are transported. Bridge pillars, where mud and silt flow past the pilings, etc., can be protected with this new technology.

2. Protective Coating for Composite Tooling - Tooling used to make composite parts are frequently made from carbon fiber composites. The prepregs containing both resin and fibers are shaped over a jig and cured to form a mold. Composites offer unique thermal expansion properties when mbsequent prepregs are cured over thein io fum production parts. One problem has been their inability to prevent scratches or galling on the surface. These surface imperfections are then transferred to the part surface.

Coatings can now be prepared with the plasma spray process to add a thin, high-modulus coating to the mold surface. Examples of typical coatings are polyvinylidene fluoride, or ultra-bigh molecular weight polyethylene, into which tungsten carbidelcobalt or magnesium zirconate is added. These coatings adhere extremely well to composite tooling, especially of the bismaleimidekarbon fiber varieties.

CONCLUSION

This new plasma spray process allows coatings to be formed directly on a substrate. Accordingly, not only can thermoplastics be applied, but high filler levels can be added to enhance and to create new performance features never obtained in practical operations. The substrate can be non-conductive and need not be heated. The thermoplastic coating is not oxidized in the process, nor is the substrate. Methods have been devised to allow the equipment to be robotically controlled.

The advantages are summarized as follows:

100% solids, no volatiles given off during processing Heavy coatings (2 m i l s to 2 inches) can be applied in single application Process is simple, easily automated Essentially 100% material utilization Low reject rate - no runs, drips, or sags

Substrate need not be conductive Substrate need not be heated Can be field applied

REFERENCES

Plastics Technology, October 1991, p. 30.

TABLE I: LIST OF SPRAYABLE POLYMERS

LPE

UHMWPE

PP

Nylon Thermoplastic Elastomer

Nylon 11

Nylon 6/12

Polyester

PVDF

PVDF/HFP

PTFE

Linear polyethylene

Ultra-high molecular weight polyethylene

Polypropylene

Polyetheramide copolymer

Flexible nylon

6,12 copolyamide nylon

Polyester

Polyvinylidene fluoride

Polyvinylidene fluoridehexafluoropropylene copolymer

Polytetrafluoroethylene and copolymers

Table 11: NOVEL COATINGS AND COMPOSITIONS

Formulations can be created to give additional performance features to the thermoplastic polymer selected for spraying. Additives can be employed to create:

e

e

a

e

e

e

e

e

e

e

e

e

e

e

Corrosion Barriers

Fire Retardancy

Abrasion Resistance

Impact Resistance

UV Stabilization

Heat Stabilization

Ozone Barrier

Electrical Conductivity

Fiber Reinforcement

Color and Pigmentation

Density Reduction

Matching Coefficient of Thermal Expansion (CTE)

Friction Reduction

Biocides

Others, as Required

FIGURE 1. m S SECITON OF THERMopLAsI?C PLASMA SPRAY GUN

FIGURE 2. HANDHELDAPPlX2?iTIONOF

COA!SING 'IO A BOAT HuL;L

.