Flame spraying thermoplastics for plastics/metal joining

TWI Bulletin, December 1987

Part 1 - Techniques and experimental procedure

by Martin Day

Martin Day, BSc, is a Research Metallurgist in the Materials Department.

Flame spraying of thermoplastics on to metal substrates has been examined, with the aim of obtaining plastics/metal joints via the sprayed layer. The experimental techniques used to assess coating soundness and bond strength for a range of plastics under various spraying conditions are described, with welding trials used to produce lap shearspecimens between plastics and metal sheet.

Techniques for joining plastics to metals have many potential applications in the automotive, aerospace and other industries where plastics are being used increasingly for structural purposes. Appropriate methods for joining suchdisparate materials have not yet been investigated in detail, but one possible approach is to use plastics coatings to provide a joining medium between the metal and plastics components. Conventional plastics welding methods ^[1] could then be used to join, for example, a thermoplastics component to a thermoplastics-coated metal article.

The requirements for plastics-to-metal joints can be identified in many areas, but one particular example is the automotive industry where plastics are finding widespread application in both the interiors and exteriors of motorvehicles. The major techniques used for joining plastics (both thermosets and thermoplastics) to metals are mechanical fastening and adhesives, both of which are used in the automotive industry. However, the techniques for weldingplastics to plastics are becoming increasingly established and it would seem advantageous to investigate methods for joining plastics to metals via a plastics coating interlayer. Accordingly, a study has been carried out on the use of flame spraying to coat metals with a range of thermoplastics which could, in principle, be used as interlayers for plastics/metaljoining.

This article details work undertaken:

1. To develop the expertise required for flame spraying plastics;
2. To identify suitable process parameters;
3. To evaluate the coatings produced;
4. To carry out trials with a view to using plastics coatings for joining plastics to metals.

Techniques for applying plastics coatings

Several techniques are available for applying thermoplastics coatings to metals, usually for corrosion resistance and good surface finish, the major ones for this purpose being fluidised bed dipping and electrostatic spraying.

Dipping

Dip coating is most commonly carried out using a fluidised bed system. The fluidised bed consists of two compartments separated by a mesh. ^[2] Cold compressed air is forced through the mesh to the compartment containing powder, causing 'fluidisation', i.e. the powder behaves like a liquid. The substrate is preheated and then dipped in the powder ( Fig.1). Each dipping causes more powder to adhere and thus the coating is built up. The product is then removed and allowed to cool, giving a coating of between 250µm and 1 mm thickness. Less frequently, dipping is also carried out using the 'plastisol' principle, in which the plastics material (PVC) is carried in a solvent.

Electrostatic spraying

For electrostatic spraying, a charged spray gun ( Fig.2) is required, to which powder is drawn from a fluidised feed hopper by clean, dry compressed air. ^[3] The metal substrate is earthed, so a high voltage electrostatic field is formed between the gun and substrate. The powder particles are then sprayed usually with no heating, becoming charged in the process,and adhere to the substrate. The next stage is to stove, causing the particles to melt and fuse together to form the coating. Only relatively thin coatings of around 50-150µm can be achieved by this technique, but there is little wastage and coatings possess uniform film thickness. Variations of this technique include hot electrostatic (flock) spraying and airstatic spraying.

Flame spraying

The idea of spraying plastics materials, adapted from metal spraying technology, has long been of interest. ^[4] The technique of thermally spraying metal powders dates back to the beginning of this century and, in 1923, Schori took out the first patent for spraying metal powders using a torch. ^[5] Had suitable plastics materials been available, this torch would have been capable of spraying them.

However, no sooner had plastics been produced in a powder form suitable for flame spraying, than fluidised bed dipping (1954) and electrostatic spraying (1962) were introduced. ^[6] Because these methods give a more uniform coating thickness, flame spraying was not favoured and, indeed, these two alternative processes remain the most popular for applying plastics coatings. Nonetheless, in recent years, flame spraying has been widely used and Hearn ^[7,8] relates several case studies from Europe, USA and Japan, in particular the prevention of corrosion in the oil industry.

In flame spraying, a consumable in powdered form is propelled through the centre of a flame on to a substrate to form a coating. The majority of flame sprayed plastics are semi-crystalline thermoplastics because amorphous thermoplastics are not readily available in the powdered form. It is difficult to flame spray thermosets, because of the curing procedures required. So the number of plastics materials suitable for flame spraying is comparatively small compared with the many available for other industrial purposes.

Initially, plastics were sprayed using equipment designed for metal powders but the need was soon identified for pistols made specifically for plastics. ^[6] In the original guns, the powder came into contact with the hottest parts of the gun, melted and burnt, ultimately causing blockage. This problem was overcome by changing the path of the fuel gas and allowing the powder delivery tube to be cooled, but the principle of flame spraying plastics remains similar to that of metals. Combustible gases surround the material being sprayed to melt and 'atomise' it, and a high velocity stream of gas, usually air, conveys the atomised particles on to the substrate. ^[7] To produce the flame, a fuel gas such as propane is used which is mixed with either pure oxygen, to give a small flame, or air to produce a more diffuse flame.

The advantages of flame spraying over other plastics coating techniques are:

1. The process is portable and versatile. Work can be performed on site and large objects may be coated without the need for large tanks or furnaces. Small components may also be coated easily;
2. Thick coatings, ranging from 250µm to over 1 mm can be produced;
3. Capital outlay is low;
4. Powder changes can be made rapidly and there is no need to use large quantities of powder, so the process is not time consuming.

Several disadvantages exist, namely:

1. There is a great dependence on operator skill;
2. The surface finish may be inferior to that produced by other techniques;
3. Wastage can be high because of overspray;
4. Coating thickness is not easily controlled in manual spraying:
5. The plastics may suffer increased degradation from overheating compared with other techniques;
6. More porosity may be obtained from flame spraying than other processes.

In large part, the disadvantages of flame spraying are not of significance in the context of achieving usable plastics/metal joints. The versatility is clearly attractive, however, while the ease of obtaining thick coatings may well be an asset in providing some tolerance to the eventual choice of welding process. On this basis, the present study used flame spraying to produce coatings on aluminium and mild steel, with assessment of the viability of the approach to produce interlayers for subsequent plastics/metal joining.

Experimental procedure

Method of investigation

Procedures for depositing visually acceptable single and multilayer coatings using a range of powdered thermoplastics were determined, and the resultant coatings were assessed by spark testing, sectioning and using a commercial adhesion tester. Using the adhesion test the influence of grit blasting the substrate before deposition was evaluated. The fracture surfaces of test specimens were later examined using scanning electron microscopy.

Welding trials were performed using the hot plate and ultrasonic processes to produce lap shear joints between coated metal and thermoplastics, which were then mechanically tested and sectioned.

Development of a spraying procedure

The thermoplastics powders investigated in the work are detailed in Table 1. Pebax and PEEK, because they were obtained towards the end of the programme, were evaluated only with respect to preheat temperature. All the powders are commercially available fluidised bed grades having a particle size range of approximately 80-200µm.

Table 1. Thermoplastics available for coating

Coatings were sprayed on to 1mm thickness mild steel and 1.2mm thickness commercially pure aluminium substrates, approximately 120 x 120mm square. Prior to spraying, the substrate surfaces were either:

a) degreased with acetone, or
b) degreased, blasted with coarse alumina (between 630 and 790µm average grit size) and degreased again.

To avoid further contamination, spraying was performed as soon after preparation as possible.

Flame spraying was performed using a Magnum spray torch ( Fig.3 and 4) supplied by Schori UK Ltd, and designed to be used exclusively with an oxygen/propane fuel gas mixture. The powder was contained in a fluidised container (0.5kg capacity) mounted on the gun. A purpose-built spray booth ( Fig.5) fitted with an extractor fan and glassfibre filters gave a clean and safe environment for spraying. Air was used to fluidise and propel the powder and also to cool the delivery tube ( Fig.4). The air supply was cleaned and dried by a filter attachment and the propane and oxygen cylinders fitted with two stage regulators.

A manual spraying procedure was developed using recommendations from the torch manufacturer as a starting point. ^[9] During these trials, the procedure was optimised on the basis of the visual appearance of the coating, the aim being to apply a smooth coating of uniform thickness.

The propane was lit using a sparker for safety reasons. Oxygen and air were then fed in to produce a neutral (pale blue) flame. After preliminary trials with each consumable, a flame of approximately 100mm in length was selected as being convenient and controllable for both preheating and spraying. For each consumable used, visually acceptable coatings could be deposited using similar gas pressures ( Table 2).

Table 2 Gas pressures used for flame spraying

To obtain adhesion of the plastics studied, it was found necessary to preheat the substrate. Temperatures above and below the melting point cited by the plastics manufacturers were investigated, at intervals of 10°C.

The temperature of the substrate was monitored by attaching platinum resistance pads, connected to a digital thermometer, to the underside of the workpieces. Because the substrates used were thin, it was assumed that the temperature reading represented that of the surface being coated. Following ignition and after appropriate adjustment, the flame was used to preheat the substrate to the desired temperature and the powder was introduced ( Fig.6) using a lever on the gun. Further layers were applied as required but, to avoid overheating, the workpiece was allowed to cool to the preheat temperature between passes because its temperature increased during spraying.

Examination and testing of coatings

The thicknesses of coatings obtained from one spray pass and five passes, for all materials except Pebax and PEEK, were estimated using a micrometer. Each coating was measured at least eight times. Two techniques were used to determine coating integrity. Firstly, an Edwards high frequency spark tester was moved slowly over the surface of a flame sprayed coating of each material. If the tester detected a discontinuity, a discharge occurred which could be observed as a blue spark. Secondly, transverse sections were taken from coated specimens and polished to a 1µm finish. Polarised reflected light microscopy was used to examine the sections.

An Elcometer 106 portable adhesion tester ( Fig.7) was used to measure the adhesion of two pass plastics coatings to steel and aluminium substrates. A small aluminium test dolly was adhesively bonded to the coating and pulled off using the tester, which recorded the force required. Several adhesives were assessed, namely Araldite and Rapid Araldite (Ciba Geigy), Flexon 241 toughened acrylic (Permabond) and Superbonder 495 (Loctite). Araldite was found to be most reliable and was subsequently used for the adhesion measurements.

Both the plastics coating and the test dolly had to be grit-blasted in order that the strength of the adhesive joint exceeded the adhesive and cohesive strengths of the coating. If the adhesive bond failed, the test was invalid according to ASTM D4541-85. ^[10] If the tester pulled the coating completely away from the substrate, it was taken as an adhesive failure but, if the adhesive strength exceeded the tensile strength of the coating, fracture occurred within the coating and was termed a cohesive failure.

After testing, selected failure faces were sputter coated with gold and examined using a scanning electron microscope (SEM).

Joining plastics to metals

Coatings of PPA, approximately 1.5mm thickness, were applied to 1.5mm thickness aluminium and steel sheet using the conditions determined already. The coated substrates were then machined to give 25 x 102mm coupons which were welded to 3mm thick Propathene (ICI polypropylene) coupons, also measuring 25 x 102mm, to give lap shear specimens.

Hot plate welding was performed on a Bielomatic machine using a hot plate, covered with a 0.4mm thickness PTFE coating, at a temperature of 235°C. It was apparent that the PPA coating melted at a temperature well below that of the Propathene, so the coating was heated at 235°C for 7sec and the Propathene for 30 sec, to achieve more equal melting. Heating and welding pressures were both set at 1.5bar and the joint was cooled after welding for 20sec. The same welding conditions were used to make a weld between two Propathene coupons, as a comparison.

Ultrasonic welding was carried out on a Forward Ultrasonic 20kHz machine using a vibration amplitude of 40µm. A steel wire (0.35mm diameter) mesh was used as an energy director and placed between the PPA coating and the Propathene. The optimum welding parameters were determined to be: delay 1sec, ultrasonics 0.5sec, cooling 2sec, pressure 1.5bar. Again these conditions were used to make a comparison weld between Propathene coupons.

Welds were shear tested at room temperature using a Hounsfield tensometer at a crosshead displacement rate of 28 mm/min. Welded coupons were also sectioned in the longitudinal direction and polished to a 1µm finish before examination under the optical microscope.

(Part 2)

References