BINDER – "BINDS" THE PIGMENT, PROVIDES ADHESION, INTEGRITY AND TOUGHNESS TO THE DRY PAINT FILM

The binder is a very important ingredient that affects almost all properties of the coating, especially:

• adhesion and related properties like resistance to blistering, cracking and peeling
• other key resistance properties like resistance to scrubbing, chalking and fading
• application properties like flow, leveling and film build, and gloss development

With no pigment present, most binders would dry to form a clear, glossy film; some binders are used without pigments to make clear finishes and varnishes.
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Oil-based Binders
Latex-based Binders

Pigment reduces the shininess or gloss of the binder. By incrementally increasing pigment levels, and by using larger particle pigments, the following gloss levels are achieved:

1. gloss (least amount of pigment)
2. semi-gloss
3. satin or "silk"
4. eggshell
5. flat (greatest amount of pigment)

Paint gloss is determined by using instrument readings of reflectivity taken at different angles from the vertical (polished glass = 100):

The gloss readings at 20° serves to describe the "depth of gloss" is used to describe gloss and semi-gloss paints. The reading at 60° is the measurement of gloss referred to most often, and is used with all but dead-flat paints. The 85° reading describes the "sheen" of flat, eggshell and satin paints.

Paints described as flat, satin, semigloss and gloss will have sheen and gloss values falling into the ranges tabulated below. This is not to say that a given product will vary within the range; rather, each value for the product will be designed to be in the range described below. For example, a particular semigloss paint might have a 20° gloss reading of 15, and a 60° gloss reading of 55.

The paint chemist uses a figure called the PVC (pigment volume concentration) to indicate the relative proportion of pigment to binder for the paint formulation. The PVC is a comparison of the relative volumes (not weights) of total pigment and binder, and is calculated as follows:

Volume of Pigments
PVC% = -------------------------------------------------------------- X 100
Volume of Pigments + Volume of Binder

Typical PVC values associated with different levels of paint gloss are:

Thus, a broad range of pigmentation levels is utilized in designing flat paint formulations. Higher quality flat paints, both interior and exterior, will generally have PVCs in the 38 - 50 percent range. Because these flat paints have more binder available per unit of pigment, they will have better durability than higher PVC flats, all else being equal, as measured by properties such as scrub resistance and dirt resistance for interior use; and color retention, chalk resistance, mildew resistance, and general durability for exterior applications. (Products designed for interior or exterior use must be used respectively.)

Painting contractors often choose more highly pigmented "dead" flat paints for new interior construction to hide unevenness of construction (particularly taped wall joints) and for their uniformity of touch-up. In exterior use, high PVC flats do not stand up as well as lower PVC formulations, particularly in freezing climates and/or in use over wood.

The gloss requirement for paints shinier than flats restricts the range of PVC that can be utilized, compared to the range available with flat finishes. Some product specifications and/or MSDS will indicate the PVC of the product.

Oil-based and Latex Paint Binders

1. Oil-Based

The binder in an oil-based coating is made from a vegetable oil that "dries," or oxidizes, and crosslinks when it is exposed to the air, and thus develops the desired properties of the paint product. Drying oils traditionally used in paints and coatings include linseed oil (squeezed from flax seed and refined), tung oil (from fruit of the chinawood tree), and soya oil (from soybeans).

Today, few paints are made with oil alone; rather, they are based on modified oils called alkyds. Alkyds dry harder and faster than oils. Some coatings, particularly exterior primers, are made with combinations of oils and alkyds to achieve appropriate flexibility. The term "oil-based" is commonly used to refer to both oil and alkyd coatings.

Film Formation of oil and alkyd-based paints is a two-step process:

When the paint is applied to a surface

1. the liquid evaporates and leaves the binder and pigment on the surface; and

2. the binder then "dries" or oxidizes as it reacts with the oxygen in the air.

It is this drying, or oxidation, that develops the hard, tough properties of the oil or alkyd paint. However, the oxidation process can ultimately cause this type of paint to harden to the point where it is vulnerable to cracking and chipping. The oxidation also causes yellowing, which typically is bleached out by sunlight, but may be quite noticeable in an area protected from sunlight, e.g., an inside room or closet, or a wall behind a picture frame.

2. Latex-Based Binders

Most water-based paints are "latex" paints*. The binder in a latex paint is a solid, plastic-like material dispersed as microscopic particles in water. This dispersion is a milky-white liquid, which is called latex in the paint industry, in that it is reminiscent of natural latex from the rubber tree. Latex is also called emulsion, and in some countries, such as England, latex paints are referred to as emulsion paints.

Except for appearance, the latex used in paint is in no way connected with the natural latex used in some kinds of rubber gloves, which reportedly have caused allergic reactions among certain users of the gloves.

The paint manufacturer makes a dispersion of the pigments which will go into a batch of paint, and adds the latex binder. Thus, the paint consists of dispersed pigment and binder, along with some additives and liquid, mainly water (see sections below).

Film formation of latex paint occurs when the paint is applied and the water evaporates. During this process, the particles of pigment and binder come closer together. As the last vestiges of liquid evaporate, capillary action draws the binder particles together with great force, causing them to fuse and bind the pigment into a continuous film. This process, called coalescence, is depicted in the following graphic:

Latex Paint Film Formation

This mechanism of film formation is what allows water-thinning and water cleanup with latex paints, while providing prompt development of a water- and weather-resistant film shortly after application. The latex paint film retains microscopic openings that allow it to "breathe", that is, allow moisture vapor to pass through. The latex paint film is thus more tolerant of moisture coming from inside the building than oil or alkyd paints, which form a "tighter" film, and are prone to blister if moisture is behind the paint, e.g., if the oil or alkyd paint is applied over damp wood or stucco.

* water-based paints that are not latex based include watercolors, poster paints, tempera, and most finger paints

On the other hand, latex paints may blister from rain, dew or other sources of water on the outside of the coating, if the paint:

• has limited adhesion capability
  
  
• was applied over a chalky or otherwise unclean surface, such that the paint's adhesion was compromised
  
  
• has not had enough time to dry thoroughly

Under these conditions, blistering tendency will be greater if the paint has high levels of tinting color.

The mechanism of latex paint film formation has some limitations. Because the binder particles are thermoplastic (tending to get softer at higher temperature, and vice versa), they will get too hard to fuse into a continuous, durable film when applied at too low a temperature. This is the main reason paint manufacturers specify a minimum application temperature (typically, 50° F) for latex paint products. And if conditions are such that the paint dries very fast, film formation and durability can be compromised, since very quick drying can reduce mobility of the particles before the film is adequately formed. Conditions that can contribute to overly fast drying of exterior paint are very high temperature, wind, low humidity, painting in direct sunshine, and painting over a very porous surface.

Types of latex binder: There are different broad chemical types of polymer used as latex paint binders. The two types used most commonly in North America are:

100% acrylic and vinyl acrylic (also called PVA for polyvinyl acetate)

The formulator has many binders of each type from which to choose. These will vary in terms of adhesion, particle size, flow and leveling, hardness, solids content, price, and other characteristics.

Assuming that an appropriate binder is used for the intended application, and that all else is equal, 100% acrylic binders generally excel in the following properties for exterior applications:

As a result of these properties and benefits, 100% acrylic latex paint is often specified for use on exterior surfaces where top quality performance is required. (Acrylic binders are significantly more expensive than vinyl acrylics.) For interior applications, acrylic binders afford benefits in terms of adhesion under wet conditions, resistance to waterborne stains (food stains like coffee, juice, wine, etc.), resistance to blocking (sticking), and resistance to alkaline cleaners. However, because of cost differences, vinyl acrylic is the predominant binder used for interior paints, with 100% acrylic types generally reserved for top of the line products. The trend is toward use of high-performance interior acrylic paints, both for highly demanding uses such as kitchen & bath, and more general applications where ease of cleaning is important.

A third category of latex binder is styrenated acrylic. Styrene is included in the binder for enhanced water resistance, gloss development and cost reduction; however, the amount of styrene that can be used is limited because too high a level can create a tendency to crack and to chalk excessively, leading to fading. These binders are used in some masonry sealers, gloss paints and direct-to-metal coatings.

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