Glass Bottle Spray Coating: Route, Tests, Acceptance

Spray coating puts a cured film, usually somewhere between roughly 15 and 40 micrometres thick, on the outside of a bottle that has already been formed. Choose it over colour melted into the glass when the shade is a brand reference the melt palette cannot reach, or when a programme runs several colours in small lots. The price of that freedom is a film that has to be specified: adhesion, abrasion, solvent rub, colour tolerance and a sampling plan should all be written down before an order is placed.

What the contents, the channel and the regulations ask of the film

Bare glass raises none of the questions a coated bottle does. Once a film sits on the surface, somebody has to know what it is made of, how firmly it bonds, what dissolves it, how close it may come to the product, and how buyer and plant will agree that a delivery matches the approved sample. The film itself is cured by heat, by ultraviolet light or by a two-component reaction, and each system behaves differently under those questions.

Projects that leave them open usually meet them later, in one of three places. At filling, a cap refuses to seal because coating has wrapped onto the top land of the neck. After transit, shoulders arrive scuffed where bottles rubbed in the partition. A year on, a retailer asks for proof that the pack suits the product, and no one can say which pigment components were ever tested. Each of these is settled in the specification, not on the production floor.

Contents

What goes in the bottle normally drives everything else. Perfume and eau de toilette carry a high share of ethanol, which attacks many decorative films. Creams and body butters are fatty, so the residue left by fingers or a shelf edge is oily. A direct-drink beverage bottle adds a different concern: the consumer's mouth touches the rim and the product flows over it. Candles and diffusers are not food contact articles, which makes the compliance side far simpler.

Sales channel

Prestige retail and duty-free counters light a bottle strongly from one direction, and film defects nobody would spot in a warehouse stand out on the shelf. Online sales depend on studio photography, which forgives micro-variation but punishes any shade difference between the hero image and the bottle a customer unpacks. Refill and bulk channels care mostly about cost and whether the bottle survives a second trip.

Regulatory frameworks

Most markets treat food-contact glass as a food contact material. In the European Union the governing rule is Regulation (EC) No 1935/2004; in the United States the matching Food and Drug Administration provisions apply. Polymeric coatings fall under their own rules, such as Regulation (EU) No 10/2011 on plastic food contact materials. Cosmetics are handled separately, for instance under Regulation (EC) No 1223/2009, where the safety assessment must consider the packaging and how it may interact with the product. These are names of frameworks. They say nothing about the status of any given supplier, and a specification should keep that distinction clear.

Four decisions, taken in order

Each step below narrows the next, which is why the sequence should not be shuffled.

1. Colour. If the target is a fixed reference with a hard tolerance, such as a shade already printed on the carton and label, ask whether the melt can reach it. Melt colours are whatever the batch chemistry yields: flint, the ambers, the greens, cobalt blue and a few more. They are stable and permanent but cannot be tuned freely. A surface film can follow nearly any reference, metallics, pearlescents and soft-touch matte included, so the further the target lies from the melt palette, the stronger the argument for spraying.
2. Lot size and colour count. Changing a melt colour is a furnace event involving a large volume of glass, a long transition and a good deal of material run off before the shade settles. Changing a sprayed colour is a finishing-line event: clean the line, load another coating, and coat the same glass in small lots. A brand launching four shades in modest quantities will find spraying far easier to schedule than a separate melt for each.
3. Where the colour must survive. Rough handling, packing without partitions, washing before filling and alcohol-based contents all raise the durability requirement, and that pushes back on the first two choices. No coating is at once the most economical, the most flexible in colour and the hardest wearing; every gain costs something elsewhere.
4. Acceptance. Before the purchase order goes out, the buyer should be able to state five things: colour tolerance, abrasion test, solvent rub, adhesion test and sampling plan. If any is missing the specification is unfinished, and a delivery dispute will turn on opinion instead of data.

Spray coating compared with in-mass colour, metallising and etching

With the container shape fixed, a buyer realistically picks between four ways of colouring or finishing it. In practice, lot size removes two of them before durability is even discussed.

Two readings matter most. A programme with more than about three colours, or with any brand-specific reference, normally points to spray coating. A direct-drink format, or a product that will sit in the bottle for years, justifies the larger melt run because it removes a whole category of risk. The comparison carries no cost figures on purpose: what should be weighed is the coating together with its acceptance testing and the risk of a rejected batch, not the coating operation alone.

Tests that belong in the purchase order

Three tests do most of the work, and each needs its parameters written out. Naming a test without them specifies nothing.

Cross-cut adhesion

A multi-blade cutter or guided blade scores a lattice through the film down to the glass, typically six cuts in each direction at one millimetre spacing. After the debris is brushed away, adhesive tape of defined width and adhesion is pressed onto the lattice and peeled back at an angle close to sixty degrees. The result is graded on a standard scale that runs from no detachment at the intersections to removal over areas larger than the cut squares.

Ask for results per container, taken at the shoulder as well as the body. The wall thins on the curved shoulder and the coating flows differently there, so a film that grips the vertical wall can still fail higher up. For softer films a wider spacing of two millimetres is sometimes used, because a one millimetre lattice can slice straight through and flatter the result.

Abrasion

There are two families of method. One uses an oscillating or rotary device loaded with a defined abrasive medium and run for a set number of cycles, reporting the change in haze or gloss, or the cycles needed to break through visibly. The other is a plain linear rub with a felt or cloth pad under a set load, counting strokes until the colour shifts or a wear track appears. Either way, fix the medium, the load and the cycle count.

Include the base in the test. Bottles ride conveyors on their bases, and the worn ring that develops there, sometimes called a burn ring, is the abrasion complaint heard most often after a filling run.

Solvent rub

This test tells a coating fit for water-based products from one fit for ethanol-rich ones. A pad of stated material is soaked in a stated solvent and rubbed over a stated area, under a stated load, for a set number of double strokes. Three things are then recorded: gloss change in the rubbed area, pigment transfer to the pad, and whether the film has come away down to the glass.

Use the real contents where possible. For an eau de toilette or alcohol-based mist, the formula itself, or an ethanol solution prepared at the formula's strength, tells far more than acetone, which defeats almost every film and so proves little. A body product is better represented by a typical oil or an isopropyl wipe. If the formula cannot be released, a standardised alcohol solution at a defined alcoholic strength is an acceptable stand-in, provided the substitution is recorded.

Supporting checks at sampling

Two further checks are worth running on samples even if they are not repeated on every batch. Accelerated ageing holds the coated piece at raised temperature and controlled humidity, then looks for yellowing, chalking or lost gloss. Product contact incubation fills the container with the real contents, caps it and holds it warm for several weeks before inspecting the shoulder and the line where the cap skirt meets the glass. That last spot reveals volatile contents that attack the film only where vapour can collect under the cap.

None of this requires the buyer to own a laboratory. It requires stating the parameters, asking which laboratory will run the tests, and asking what the report will show. A bare pass or fail, with no medium, load or cycle count, cannot be compared from one batch to the next.

Colour tolerance and batch acceptance

Shade is the subject disputed most often on sprayed glass, and nearly every dispute starts from a colour described in words where numbers were needed. Five elements close that gap.

• A master sample. At sampling, both sides agree a physical reference. Each keeps one panel or container, labelled with coating reference, batch and date, and stored away from light and heat. Every delivery is judged against the master, never against the delivery before it; chaining comparisons lets drift build up until a batch that would fail the master passes its predecessor.
• Instrument readings. Colour is reported in CIELAB, meaning a lightness value and two chromatic axes, with the gap from the master given as delta E. For a solid opaque coating a common commercial tolerance is a delta E of about 1.5 inside one production batch, somewhat wider between batches. Anything tighter than about 1.0 generally needs a deliberately controlled operation and should be discussed before it enters a contract. Metallic, pearlescent and other effect finishes shift with viewing angle, so delta E alone is not enough: add a gloss unit target at a defined angle and a multi-angle colour check.
• Light source and geometry. Two bottles that agree under warm shop lighting may part company in daylight, and coated clear glass looks different in transmitted and reflected light. Measure a translucent tint both ways, and do the visual check in a light booth under a defined illuminant such as D65 against a neutral grey surround. Translucent colours are also the hardest to hold, because the shade depends on what lies behind the glass: one coating on flint and on amber bottles will not read alike.
• A defect list. Keep it in the same document as the colour tolerance. At a minimum it should set limits for sags and runs, orange peel, dust and fibre inclusions, fisheyes and craters, edge pull-back at masked zones and shade variation within a single container. Give each a size or area limit and a permitted count per container, and sample under a recognised scheme such as ISO 2859-1 at an agreed inspection level and acceptance quality limit. Otherwise one inspector's opinion rejects a batch and another's defends it.
• Time since coating. A film goes on changing slightly after the oven, and some two-component systems cure for days. State the interval between coating and testing in hours or days, and apply the same interval to the master. Measuring a batch on coating day against a master measured a week after proves nothing.

Masking a clear window or an uncoated neck

The film does not have to cover everything. Partly coated bottles are common in cosmetics and spirits; the effect is made by masking chosen areas before spraying and removing the mask afterwards. Four methods are in regular use:

• a silicone plug or cap pushed onto the neck, shielding the finish and the sealing land;
• a heat-shrink sleeve around the body, leaving a defined band bare;
• a water-soluble masking lacquer, printed or dipped in a pattern and washed off later;
• a rigid template or stencil held against flat or simple shapes.

The shape of the bare area decides which one fits. A straight band is simple and a curved window across a shoulder is harder. Hardest is a window that has to line up with later decoration or a label, because the mask's positional tolerance then becomes a print registration tolerance.

The commonest reason to mask is functional. Coating on the top land of the neck finish is a defect: the film is softer than glass, takes a compression set under the cap liner and opens a leak path. Many buyers therefore require the whole finish, top land and thread included, to be free of coating, with the coated zone starting below a set line on the shoulder. Dimension that line on the drawing. A spoken request to "keep the neck clear" gets read generously on the line.

Masking brings three penalties. It inserts a manual or semi-automatic step into finishing, which adds to the cost of each piece. It leaves a visible step at the edge of the coated zone, roughly as high as the film is thick and more noticeable with soft or heavy films. It also has its own defects: mask lift, where film creeps underneath and leaves a ragged edge; mask residue, where release agent stays on the glass; and scratches from handling as the mask comes off.

When the aim is simply to show the contents, a masked window is rarely the best design. Leaving the body transparent and colouring only the shoulder and base, for example, shortens the masked perimeter, keeps the label area free and makes the edge easier to hide behind a decorative line.

Where an exterior coating sits relative to food contact

Arguing over the word "compliant" gets nowhere. Marking zones on the drawing does. Three are needed:

• Product contact zone: every surface the contents can reach. On an ordinary bottle or jar that means the inside wall, the bottom, the neck bore and the top land where the liner seats.
• Lip or mouth contact zone: relevant to direct-drink containers, where the consumer's mouth meets the glass.
• Exterior decorative zone: everything else.

A coating confined to the exterior decorative zone is not part of the product contact surface in a normal closure design. That is the reasoning behind its use on food and beverage packs, and it depends on conditions that have to be verified:

• no coating inside the container, including overspray drifting into the bore while the neck is open;
• none on the top land or thread, where contents can creep past the liner during filling, transport or storage;
• no migration through the glass. This is not a realistic worry for a fired, ceramic-like film, yet it deserves an answer when a plasticised organic film spends a long time against a strongly solvating content;
• no removal by the process, for example by a solvent carrying pigment onto a surface that later meets the product.

Documents to request

The paperwork comes from the coating supplier, not the glass plant, and it should be specific to the colour. Three items answer most requests: a declaration of compliance under the European framework for the relevant material type, a migration test report using a simulant suited to the intended use, and a heavy metals statement covering the pigment package. Per-colour documents matter because pigment loading varies by shade. A bright red, a yellow and a deep blue may rely on different pigment chemistries, so a document issued for black tells you nothing about red. If the outer surface must suit indirect food contact, say so at enquiry, since that narrows the coating families that can be offered.

Wording to avoid and wording to use

Neither side should claim that a container is "food grade", because that describes a material in an application and not a container. Nor should anyone claim approval has been granted unless a specific certificate exists for that product and use. The accurate statement is that the exterior coating lies outside the product contact zone, that the coating supplier holds a declaration of compliance for the specific colour, and that its reference will accompany the delivery documents. Every part of that can be checked.

Cosmetic packs

Under Regulation (EC) No 1223/2009, assessing how product and packaging interact is the responsibility of whoever places the product on the market. For the buyer this means building the evidence trail, not citing one. A coating outside the product contact zone is easy to justify, as long as the file records the coated zone, the closure design and the incubation result.

Filling line and packing checks for a coated bottle

With the route settled, four points on the line determine how smoothly the project runs.

• Sealing land. Leave the top land bare, and check the closure's inner surface against actual glass, not a drawing. Film on the land alters liner compression; the symptom is leakage or a cap that never reaches its torque target.
• Thread fit. Film adds thickness. On a fine thread such as a standard continuous-thread neck, that can change how the cap engages: a stiffer or looser turn, or at worst a cap that binds before seating. Keep the thread uncoated. Where the design wants colour near the neck, set a line on the shoulder and leave the finish clear.
• Conveying. Coated glass slides differently from bare glass, soft-touch matte most of all, so guide rail pressure and transfer points may need resetting. Inspect the base ring for wear after the first thousand units. A coated base is sometimes specified to reduce the problem, but it then acts as a wear part and should be assessed as one.
• Packing. Hard partition edges can rub through the film at the widest point of the body, and a paper interleave can press fibre into a matte surface. Before the first bulk order, test a full shipper, not a single bottle, under a recognised transit test protocol, and open it at destination instead of at the dispatch warehouse.

Light is the reason for opening the carton where the goods will be sold. Scuffs that go unnoticed in a warehouse often become obvious in daylight on a retail floor.

Related decisions covered elsewhere

Everything above concerns one operation, a coloured film on the outside of a glass container, and the rules for specifying and accepting it. Four neighbouring questions have their own guides.

• Choosing a surface route for a black fragrance pack, with its sourcing logic and positioning, is treated as a whole programme in our guide to matte black perfume bottle finishes.
• Frosting shares equipment with colour spraying but aims at light diffusion, so it is accepted on haze and gloss instead of shade and abrasion. The comparison of acid etching with sprayed frost is in the guide to frosted glass jar finishes.
• A black container can come from a dark melt, a heavy exterior coat or both, and the choice alters the defect list and the way colour is measured. Opacity, depth and acceptance of dark batches are covered under black glass jar specifications.
• When the choice is between a sprayed amber effect and true amber glass, the achievable melt shades and their light transmission are set out in the guide to brown and amber glass bottles.

Frequently asked questions

How does spray coating differ from in-mass colour?

In-mass colour goes into the batch before forming, becomes part of the glass and cannot come off. A sprayed colour is a film added to the finished container and then cured. The first needs a large run and offers few shades but has no film that can fail. The second suits small lots and nearly any shade, and in exchange makes adhesion, abrasion and solvent resistance items to specify and test.

Can part of a sprayed bottle stay clear?

Yes. Masked areas stay bare, which is how view stripes and label windows are made. More often the purpose is to keep the neck finish, thread and top land clean so the closure seals. Expect an extra finishing step, a visible edge where the film stops, and mask-related defects such as ragged boundaries.

How is adhesion checked before shipment?

By a cross-cut test: a lattice scored through the film, tape pressed on and peeled away, and the result graded against a reference scale. Have it done on the shoulder as well as the straight wall, because the two can behave differently.

Which solvent should the rub test use?

The one in the product. An alcohol-based fragrance calls for the formula or an ethanol solution at its strength; a body product calls for a representative oil or an isopropyl wipe. Define the pad, load, stroke count and area, then record gloss change, pigment on the pad and any film loss.

How close should batches match in colour?

Measure in CIELAB and report delta E against the approved master. About 1.5 within a batch is a usual commercial figure for solid opaque coatings, with a little more allowed between batches. Effect finishes also need a gloss target and a multi-angle check.

Is an exterior coating a food contact material?

Not when it is confined to the exterior decorative zone, stays out of the interior, the neck bore and the top land, and does not migrate. That boundary has to be drawn, verified and backed by per-colour documents from the coating supplier.

At what quantity does spray coating make sense?

No single threshold applies. The answer turns on how many colours the programme has, whether any is a brand-specific reference and how much flexibility is worth to the buyer. Compare one large melt run in a single colour with several smaller finishing runs, each needing its own line setup. As colours multiply and the quantity per colour falls, spraying becomes the more practical route. Costs are quoted per project and depend on the coating system, masking, colour count and quantity.

To have a route and a written acceptance standard proposed, send us the target colour reference, the planned quantity per colour, and whether the container or its rim touches food or is a direct-drink format.