A simple model to describe relationships between gloss behaviour, matting agent concentration and the rheology of matted paints and coatings

https://doi.org/10.1016/S0300-9440(01)00206-5Get rights and content

Abstract

A set of equations are described which relate the effect of matting agent concentration on gloss reduction to volume concentration and an intrinsic efficiency term. The latter characterises the effectiveness of the matting agent. The connection to surface roughness is described, and the parallel with equations used to predict relative viscosity of suspensions is indicated. The potential usefulness of the approach in the construction of matting curves or in the determination of critical pigment volume concentrations is discussed.

Introduction

It is widely known that particles of the order of a few microns in size are capable of producing matt surfaces when added in sufficient concentration to coatings that undergo large reductions in volume during the drying and curing process. Silica gel matting agents are widely used within the coatings industry for matting purposes and are regarded as highly efficient in producing the type of surface roughness that is responsible for the appearance of mattness.

Additional advantages of silica gel matting agents are that they are available in a range of particle sizes, pore volumes and surface treatments to suit specific application needs, are easily dispersible in a wide range of binder media and have a refractive index that is close to that of many binders, allowing clear matt coatings to be developed. These issues have been extensively discussed in the literature [1], [2].

From the matting point of view, these materials are commonly regarded as being less effective in coatings that undergo little shrinkage during film formation. Examples of these latter coatings include powder coatings, paints systems having very high solid contents and some types of UV curing systems.

Sufficient matting of these types of coating systems with conventional matting agents can sometimes be realised by appropriate selection of particle size. Rheological problems may occasionally occur, however, if matting is attempted by the use of high addition levels. Further developments are needed to handle the matting of modern coatings efficiently.

A model is being developed with the intention of explaining the influence of matting agent concentration on the gloss and rheological behaviour of coatings. The purpose of the present paper is to introduce this approach with respect to some aspects of shrinking systems. The paper will begin with a brief review of previous work carried out on the mechanism of matting by particles. Statistics that are used to characterise rough surfaces and their relationship to measured gloss values will also be considered.

It will be shown how the classic parameters of pore volume and particle size can be interpreted within the framework of the model in terms of an equation fitted to gloss-matting agent concentration data. The utility of the equations discussed to characterise matting agents and to predict gloss levels will be described and it is hoped that this will be of value in the development of matted paints. More detailed accounts and the extension to newer coating types whose behaviour during film formation differs from conventional paint systems will be the subject of later publications.

Section snippets

A brief survey of previous work on the mechanism of matting of paint films by particles

It has been realised for a long time that the surfaces of matt paint films are rough and that the perception of a surface as being matt is associated with this surface roughness [3]. The phenomenon of matting is commonly regarded as being due to a reduction in the amount of specular reflected light as a result of the surface roughness [4].

The role played by particles in the matting of paints that undergo shrinkage during drying and curing has been discussed by Field [5], following earlier work

Statistics used to describe rough surfaces and the relationship to gloss

In general, characterising surface roughness involves a consideration not only of the spread of heights about a reference plane, but also the variation of heights over the surface as discussed by Ogilvy [8].

Outline of the basic equations for describing gloss and viscosity

If g is the gloss at a given matting agent concentration c, expressed in terms of the volume fraction, and g0 the gloss of the unmatted coating, then it is proposed that the incremental change in gloss δg, for an incremental increase in the concentration of matting agent δc, can be written asδg=g−[g]δc1−kgcwhere [g] is an effectivity factor, and kg a factor to allow for the fact that each incremental addition of matting agent is made to a continuously decreasing proportion of binder. If this

Experimental

In most work, an unpigmented nitrocellulose lacquer having a solid content of 25% by weight was used for the purposes of assessing the utility of the above equations. This was regarded as a model system and was easy to work with. In some cases, a standard unpigmented two-component polyurethane coating having a solid content of 49% by weight was used for comparative purposes.

Varying addition levels of silica gel matting agents were dispersed into the above resin solutions by means of a Dispermat

Properties of the selected silica gel matting agents and determination of the fractional volume concentration and expected kg values

As described earlier, there are a large number of silica gel matting agents available differing in particle size, pore volume and surface treatment. Five silica gel matting agents were selected for the purposes of the present work differing principally in pore volume and particle size and these are referred to simply as silicas 1–5. Their properties are given in Table 1.

Silica 5 had a lower pH than the other four silica types. pH in this case refers to pH of an aqueous slurry. Pore volumes were

Results and discussion

The following summarises the results obtained with respect to gloss measurements for the various silicas as a function of fractional volume concentration and dry film thickness. Surface roughness measurements and the relationship to the measured gloss values are also given. Some preliminary results with viscosity measurements are reported.

Conclusions

The basic model concerning the mechanism by which particles matt paint films can be maintained, although allowance should be made for the dependency of the film thickness at which roughness begins to develop, tg on the initial wet film thickness tw or equivalently the dry film thickness td. tw and td are related by the volume solids of the paint together with the loss of volume that may occur due to processes involving chemical reaction and film curing.

In agreement with theoretical expectation,

References (10)

  • H. Schneider

    Matting of modern surface coatings

    Surf. Coat. Int.

    (1994)
  • H. Schneider, Micronized porous silica gels, modern matting agents for the coatings industry, Eur. Polym. Paint. Col....
  • M. Hess

    The relationship between gloss and surface roughness of paint films

    J. Oil Colour Chem. Assoc.

    (1956)
  • R.S. Hunter, R.W. Harold, The Measurement of Appearance, Wiley, New York,...
  • R.J. Field, Matt paint films: real-time observation of film shrinkage and associated roughening of the surface during...
There are more references available in the full text version of this article.

Cited by (0)

View full text