IN SEARCH OF MANUFACTURING EXCELLENCE. SIMPLIFIED ENAMEL SOLUTIONS. PICKLE FREE FOR ALL FINISHES
Jack Nicholson - Escol Products, Great Britain

Abstract
The need for improved manufacturing techniques in the production of domestic appliances is identified. A number of key issues in enamel ware manufacture are cited such as market requirements, environmental aspects, process options, and finally the development of simplified enamel systems. Namely one coat pickle and nickel free enamel systems for all finishes.

Introduction
Vitreous enamel remains in terms of resistance to corrosion, wear and colour stability perhaps the best way of protecting and enhancing the appearance of articles made from steel sheet.
A range of products from hollow-ware to cookers, building panels and grain silos are protected and decorated in this way.
Despite major advances in enamel technology some end users, particularly in the appliance industry, would, if given a choice, prefer alternative finishes such as plastic, paint, glass or stainless steel.
Those options are perceived to be less complex and more easily automated than enamel processing.
As a result, year on year, we have seen market contraction and the replacement of traditional enamel applications.
In the U.K., cooker exterior side panels, oven and grill door exteriors and washing machine tubs have, in the majority of cases, changed to other materials. So what is needed to reverse the trend?

Let us examine an appliance manufacturers requirements for the future

Is it vision or fantasy?

Recent developments and latest trends in method simplification
In a lot of areas significant improvements have been effected, usually when there has been close collaboration between all parties or any combination of end user, steel producer, application equipment provider and enamel supplier.
During the last decade we have seen major advances in all areas of enamel processing.

Fabrication
The manufacture of ovens or grills using "hydroform" technology yields a range of benefits:

Pre-Treatment
Elimination of pickle or full nickel pre-treatment has been necessary due to:

Introduction of spray and immersion wash lines utilising a combination of alkaline or neutral cleaning followed by rinse and dry stages. Eliminates priority environmental issues whilst providing pristine surface for enamelling.

Enamel Application
Following elimination of pickle:
Dark colours - pickle free wet and nickel powder or
White or Pastel Colours - 2C/1F: wet/wet
wet/dry
dry/dry

Application Technology

All systems available as integrated options combining cleaning and coating application.

Given the possibility of 2C/1F solutions, identified above, why is there a demand for single coat 1C/1F capability?
There are two primary reasons:

2C/1F - Two Coat/One Fire
E/S - Electrostatic
1C/1F - One Coat/One Fire

Colour finish flexibility
In our home market we have seen considerable growth in the sales of domestic appliances (white goods) over the last five years, reversing the trend of the early 90's. The market for gas and electric cookers has been stimulated by the offer of a wide range of colours and finishes providing ever more choice for the customer and consumer.

The range of colours included Cypress Green, Blue, Mahogany Red (Cadmium free), Mellow Yellow, Burgundy, and a number of pastel shades, such as Almond, Champagne, Lunar Green, Linen, as well as the more traditional White, Black, Coffee and Cream.
Most, if not all of these finishes would conventionally require full pickle and nickel pre-treatment for good adhesion and surface finish in a one coat system.
The application system for the new enamel colours required, where possible, automatic quick colour change systems to operate Just-In-Time principles to minimise work in progress stock levels.
An additional requirement for the U.K. market are self clean ovens or oven linings. The product design may be based on a fixed oven cavity construction in a two coat/two fire enamel system or a one coat removable lining attached to the primary and/or a second double oven specification.
The two requirements of a double sided grill pan in a range of colours and a single coat self clean system could only be achieved in a pickle free solution by conversion from one coat pickled to a 2C/2F process utilising a first coat of groundcoat for adhesion, or the provision of further spray stations for the application of the groundcoat element of 2C/1F proposal.
The challenge for the enamel supplier was to develop enamel systems that could operate pickle free in all colours including self clean finishes, and eliminate further cost for the manufacturer in the form of a capital spend for additional application plant or the additional manufacturers costs of a two coat process.
As is often the case, the manufacturers used different techniques for enamel coating, including electrostatic guns; hight speed turbo disc; and dry electrostatic powder; dipping; flowcoating.
Materials would have to be developed for each of these systems. The demand on ESCOL PRODUCTS was initially to develop one coat enamels in all finishes wet and powder for application onto a degreased only substrate using existing equipment and current steel specifications.
Secondly to evaluate the possibility of application direct onto components straight from the press shop.
Only powder to date.
The development programme encompassed four stages.
Firstly - the conversion of all dark or strong coloured finishes.
Secondly - the conversion of all pastel colours.
Thirdly - the possibility of pickle free white and selfclean coatings.
Fourthly - the evaluation of application to components direct from press or welding shops.

Pickle free development - Colours other than pastel or white
The productionising of dark adherent black, brown and blue black groundcoats has been established since the early to mid 80's.
The development of bright green and blue enamels in non pickle form proved more difficult. Chrome free formulations had to be developed for green options especially for dry powder types and for blues low cobalt materials were developed with reduced surface reactivity problems and/or sensitivity to cleaner residues or oil.
At the fineness specification required for optimum disc spray performance, it was surprisingly observed, many enamel formulations previously considered unsuitable for direct on pickle free use, exhibited more than adequate adhesion.
As a consequence, less expensive materials could be selected for further development. Additionally, it was found relatively easy to move colours in the mid range green, blue, grey and fawn brown to meet the non pickle requirement. The same formulations milled at conventional electrostatic fineness levels would not adhere when applied degrease only to zero carbon or IF steels.

Pastel colours
The development of degrease only one coat systems, for colours with L value higher than 70%, was always going to be more difficult. For a full pickle system,
colours that include honeymink, beige, champagne, lunar green and blue are usually formulated in semi to three quarters opaque titanium frit. These include for wet application between four and seven percent colouring pigment at the mill.
The above colour range could not be easily formulated for dry electrostatic powder due to potential re-cycle problems and subsequent colour instability.
Best results are achieved by the use of self tint compositions with the frits as near as possible to the final target shade.
Major difficulties were encountered using these semi/three quarter opaque titanium based frits for application direct to degreased only substrates even when fire ground.
A combination of low metallised frits, refractory and opacifier (not titanium pigment) mill additions served to produce lighter shades with adhesion properties than would be the case at normal dip or electrostatic spray fineness.
The difficulties of achieving excellent adhesion when applying semi-opaque and super opaque titanium enamels is well understood by enamel formulators.
However, effects on opacity and chemical resistance by titanium inclusion in the formulation is particularly desirable.
Hence the well established use of high titanium enamel especially for exterior coverage of cooker hob tops; and other components where brilliant white systems with high reflectance and gloss are required.
Initial results on pastel enamels were disappointing and the relationship of titanium levels on opacity and adhesion were examined. Mill added titanium dioxide also had a very negative effect on fired adhesion.

Cerium glasses with good gloss and opacity have been available since 1940's but these too exhibited similar problems of adhesion when applied direct to pickle free steels.
The incorporation of high levels of lithium to improve surface reactivity and assist bonding had a similar effect as both the titanium and cerium formulations. Higher solubility of the titanium and cerium and hence reduced opacity occurred but some gain in adhesion promotion. The impact of particle size of raw batch materials on frit opacification and the role of fluorine on opacification and adhesion was studied.
In varying the constituents of the glass each change was carefully monitored for adhesion.

Pickle free white
By 1997 a breakthrough was achieved when the first "off white" product with excellent adhesion was achieved.
This material showed lab values of L 76.09;A - 0.47;B + 3.32 and gloss level of 55%. The step change in adhesion improvement was obtained by the inclusion in the system of a special very low melting point frit.
This component was designed to preferentially react with the steel substrate before opacification developed from a second high opacity frit. This dual phase system included mill added refractory opacifiers.
By optimisation of frit formulations and mill added refractory opacifiers results were near to super opaque titanium white opacity performance but with
excellent pickle free adhesion properties. All other physical and chemical resistance values were maintained in the new formulation.
The new pickle free whites exhibited typically an L value of 88%.
Successful pre-production trials were established before "live introduction" of the new material followed in July 2000.

Pickle free (PF) self clean development
Following successful formulations of PF pastel and PF white enamels as a manufacturing concept, the technology was quickly transferred to the production of a corrosion restraint self clean system.
In the U.K. the principle of easy-to-clean (E.T.C.) enamels is considered more of a concept than a practical alternative to catalytic selfclean coatings.
Although, clearly more chemical resistant coatings offer improved soil resistance the marketing of E.T.C. products will prove most difficult in our home market. The danger is that E.T.C. enamels may not meet the consumer aspirations of a "wipe clean" performance. Still today, self clean ovens and self clean second ovens remain a major share of the total UK market.
Although these catalytic coatings have limitations in general they continue to satisfy domestic requirements.
Until recently these applications have been over a previously glossy or refractory groundcoat for ovens, or applied direct to a full pickle/nickel coated substrate.
The introduction of our pickle free selfclean option was made in January 2000 after a two year "field trial" exercise.
The exercise concluded that the PF option outperformed the established pickle/nickel system in all areas. As a consequence the application to fixed ovens is now under serious consideration.

On line performance results and conclusion
The objective of providing 1C/1F (one coat / one fire) enamel systems to enable use of existing steel substrates and application equipment has beensuccessful. Production proven products include.

Brilliant green, blue powders
Pickle free high gloss AR (Acid Resistant) Green and Blue powders

Pastel or exotic coloured enamels

Diverse range of coloured enamels for pickle free, wet application. All colours formulated utilising "user" and environmentally friendly oxides.

Catalytic self clean
Wet application
One coat pickle free catalytic self clean enamels production proven to exhibit excellent adhesion, cleaning, and absorption properties whilst providing exceptional corrosion resistance and abrasion resistance. Can be applied using any standard technology dip; flowcoat; spray (auto E/S, disc, gun bell) In use since 1999. Infinite flexibility from this development.

Electrostatic powder
One coat PF self clean powder with excellent adhesion, cleaning, corrosion resistance and abrasion resistance.

White enamels
Wet application
One coat pickle free white covercoat enamel for cooker exterior parts on line since June 2000. Alternatively it has been used as a white groundcoat for 2C1F applications since 1996. Improves brightness of final coat whilst eliminating "black" specking or groundcoat pull through.
Offers advantage over coloured groundcoats.
Under evaluation as "white" groundcoat for bath and shower tray applications.

Electrostatic powder
One coat pickle free white powder is under production evaluation. Also to be considered as a replacement for dark coloured groundcoat powders.

Degrease elimination
Special high wettability enamels exhibiting low surface tension properties havebeen milled with additions of environmentally friendly emulsifiers and surface active agents.
Following application to steel plates oiled with suitable water soluble lubricants good enamel to steel bond and surface has been achieved.
Process still to be optimised for maximum forming properties of press lubricants and enamel formulation but results confirm the potential of application of powder and wet enamels to oiled substrate.
For very difficult press lubricants a single stage auto water wash followed by drying provides a low cost environmental solution.

Pickle and Pre-treatment - Progressive Elimination

Advantages to end user

We have moved some way towards achieving our customers vision and aspirations of enamel process simplification. Further work continues.

figure 1 - Overview of enamel coating properties

figure 2 - Range of pastel colours

figure 3 - Pickle free white grill pan

figure 4 - Effect of grinding fineness on fired adhesion

figure 5 - Pickle free white for 2C/1F on non premium steels