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How we test the density of Resene paint

The many different departments at the Resene production facility in Naenae are always bursting with activity, especially the busy laboratory. Clever scientists like Hamid are responsible for not only formulating the paint, but testing every batch to ensure that what goes out the door is always of the finest quality. One of the key parts of the process is to test the paint density, which Hamid does with his handy tool – the pycnometer.

How is paint density tested?

In basic terms, density is defined as the mass of a unit volume of a material at a specified temperature. So to ascertain the density of paint, Hamid uses his pycnometer to measure the paint weight per litre. The cylindrical cup (which can hold 100ml of paint) is first weighed, then filled with paint and weighed again – after removing the extra loaded paint which is expelled through a small hole in the lid.

The difference in kilogram weight multiplied by 10 gives the all important measurement here: WPL (weight per litre). WPL is an important verification test and can indicate batch quality. If the WPL does not fall within the specifications, then there’s a good chance there may be an error in the batch.

How do we measure paint thickness?

Paint thickness is measured in microns (μm) – a seriously small measurement scale! Also called micrometres, they are used in measuring small particle sizes, thin materials such as paint coatings or even plastic sheet thickness. A micron is one-millionth of a metre, or easier to comprehend, 1/1000 of a millimetre. For context, the diameter of a single human hair is 20-40 μm, a spider web silk thread is 2-3 μm and a human red blood cell 5-10 μm.

A mere 50-100 μm of paint is expected to give protection against rust and the elements to our most prized possessions, such as our homes and cars.

Paint comes in a huge variety of types ranging from the early Resene Stipplecote, which was based on cement, to the latest waterborne enamel paints such as Resene Enamacryl.

A normal coat of Resene Hi-Glo applied at 12 square metres per litre will give a dry film build of 35 μm thick. If you had a 1mm crack in a concrete wall, it would take about 28 coats to bridge the gap!

To begin the process of testing paint density, Hamid mixes the batch he wants to test.

He then tips 100ml of paint into the pycnometer.

What is in a can of Resene paint?

To create a quality paint, quality raw materials are essential, but so is the skill of the chemist in deciding which raw material is the most suitable for the desired purpose. Raw materials are often interdependent, which means they will not work well in the paint unless accompanied by other critical raw materials. If you’ve ever wondered more about the science behind Resene paint, read on to expand your mind!

The main components of Resene paint are:

Binders: These work to stick the paint together, form a solid film and give adhesion.
Pigments: These colour the paint, create opacity, control gloss, prevent corrosion and add bulk and other physical, mechanical and chemical properties.
Solvents: These make paint usable.
Thickeners: These improve paint application and maintain its homogeneity.
Additives: These are used a little, but hugely impact product quality.

Binders

The role of the binder is the most important and it has three main jobs: To provide adhesion to a substrate, to form a continuous film, and to bind the pigments uniformly into the paint.

Pigments

A pigment may be defined as a solid, insoluble material that is added to a binder to produce colour, create opacity, control gloss, provide physical properties (such as the ability to sand the coating), reduce permeability to moisture, absorb UV, produce texture or even act to prevent corrosion.

They’re classified into three main groups:

Prime pigments provide colour, either inorganic or organic.
Extender pigments control gloss level, texture, etc.
Anti-corrosive and special function pigments.

Paint thickness is measured in microns – one-millionth of a metre!

Each can of Resene paint contains five key components: binders, pigments, solvents, thickeners and additives.

Solvents

A solvent’s most basic job is to make the paint easy to apply. They’re liquids that can dissolve other substances in the same way you dissolve sugar into your cup of tea.

When we speak about solvents in paint, we are often excluding water and referring to the organic solvents. Solvents are guest materials and once the paint is applied, they leave the system.

Thickeners

One of the most interesting misconceptions about paint and quality is that the thicker a paint is, the better the quality. Funnily enough, the exact opposite may be the case, especially if a large volume of paint thinner is needed to get the paint to a usable consistency.

Be aware that there is nothing natural about the thickness of a paint. All paint is artificially thickened and we can make most paints as thick or as thin as we like.

Paint thickeners have two main roles. The first is to prevent the paint solids from settling out in storage and to prevent the paint from sagging during application. The use of too much thickener will result in poor flow and the paint being full of brush marks. Their second job is to make the paint harder to spread and to impart what we call ‘brush drag’ to help whoever is doing the painting to apply the right thickness.

These thickeners also act to minimise roller fly-off (paint spattering everywhere when applied with a roller). Waterborne paints use special thickeners called rheology modifiers, which give excellent flow and levelling and also impart brush drag. When thinners are added to paint, the carefully engineered designer flow properties may be lost and the paint may run off the wall or sag.

Additives

Additives are all the ingredients Resene adds to paint to help the product to last longer or work better. These include:

Silicones and waxes to give slipperiness and mar resistance.
Fungicides.
UV absorbers to protect from Australasia’s harsh elements.
Additives to prevent in-can corrosion.
Deodorants to mask nasty odours.
Moisture scavenging additives to prevent certain paints gelling or gassing.

The Resene Technical team are constantly checking paints for quality and looking for new raw materials to keep improving the product. Their job is never done!

Click out our article to find out more about how Resene paint is made.