Formulating of latex paints using the relationship between PVC and CPVC

The formulation of latex paints, as well as paints in general, requires extensive knowledge of the basics of the raw material composition of paints, the raw material base and the amounts of use of individual raw materials. While the number of recommended dosages can be determined at the beginning from the doses oscillating in the ranges given in the technical sheets of individual raw materials, especially additives, other raw materials usually require skillful knowledge in building recipes, which will to a large extent constitute the basis for further modifications on the level of slight changes.

This article introduces the topic of formulating latex paint recipes with the use of solids such as PVC, CPVC and Q. Please note, however, that this is only an introduction for beginners and applies only to the principles used in the formulation of architectural water-based paints, i.e. wall and ceiling latex paints, some acrylic waterborne 1K paints DIY for wood, etc.

Introduction

The basis for building recipes in such a way of their development is building the core of the formulation based on PVC and derivatives such as CPVC and the ratio of PVC to CPVC. But what are these parameters and how are they defined?

PVC, pigment volume concentration – ratio, expressed as a percentage, of the total volume of the pigments and / or extenders and / or other non-film-forming solid particles in a product to the total volume of the non-volatile matter [1].

Fig. 1 Formula for calculating PVC

where:
ΣV_fillers – sum of the volumes of all fillers (extenders and pigments),
ΣV_binder – volume of film-forming substance (dry part of the binder)

CPVC, critical pigment volume concentration – value of the pigment volume concentration at which the voids between the solid particles which are nominally touching are just filled with binder and above which certain properties of the film are markedly changed [1].

Fig. 2 Formula for calculating CPVC

where:
OAI – oil absorption index of filler (extender or pigment)
ρ_filler – density of filler (or pigment), g/cm³

There is a direct relationship between PVC and CPVC, which allows to characterize whether PVC is larger or smaller than CPVC, which allows to determine some properties of the coating at the beginning, and these relationships are shown in Figure 4.

Fig. 3 Three levels of relationship between PVC and CPVC in latex paint formulations

Using the relationship between PVC and CPVC is possible only with waterborne latex paints for walls and ceilings, where it is possible to prepare paints with PVC below and above the PVC value. In the case of latex paints, PVC can also be equal or practically equal to CPVC, which also allows the formulation of paint recipes with specific properties.

The Q factor, also often described as lambda (λ), is the ratio of PVC to CPVC and describes a very important characteristic of the formulation, namely the porosity of the coating in terms of filling it with fillers, binder and entrapped air. Water-borne paints based on latex emulsions, especially wall paints, are an example of formulations in which it is possible to prepare recipes so that PVC is larger than CPVC.

If PVC > CPVC then Q is also greater than 1 (e.g. PVC 82%, CPVC 67%, the ratio Q = 1.22), which means that the coating will be porous and it will contain trapped air. Such a relationship, where Q is greater than 1, means that the coating will be poor in terms of scrub resistance, but also well permeable to water vapor. In the case of PVC < CPVC coatings are non-porous, and the gloss begins to gradually increase (assuming no gloss reducing fillers are used), as well as the water vapor permeability, which at some point is already kept at a constant, low level. Then Q is less than 1, which means no porosity of the coating, as the fillers are coated with a dry film forming substance (binder).

A special case is when Q is equal to 1, which means that PVC = CPVC. Computationally, it is easy to obtain such a relationship, but in practice this point is quite mobile and you can only oscillate around it. This is due to the influence of the oil absorption determined with the specified accuracy in the determination of CPVC, as well as the influence of the accuracy of weighing raw materials on the actual PVC level obtained. However, obtaining PVC as close as possible, practically equal to CPVC allows to use this relationship when Q is also equal to 1, e.g. when formulating paints with a very high hiding power index, which can be particularly used by selecting the appropriate grain size of carbonate fillers [2].

What do they mean in practice and why are they so important?

The formulation of paints with the use of PVC is not really based on building the composition of the recipe (in kg, L, lbs, %, etc.) but on building the volumetric composition of the dry coating. Building a formulation in this way allows to determine the amount to be used in the recipe, but on the basis of determining the volumetric composition of the coating, where PVC refers to the percentage by volume of all pigments and fillers in the volume of the coating. Then, for example, PVC 60% means that fillers (and pigments) constitute 60% by volume in the dry coating, and 40% is a dry part of binder (dry volume of film-forming substance), which binds everything together.

Therefore, building a recipe to achieve PVC 60% consists in such balancing of raw materials – at the beginning, for example, with a filler and a binder (polymer dispersion) to achieve the desired PVC 60%, which results in a specific proportion of individual components, here the filler and polymer dispersion, in one calculation.

Determining the level of PVC at the beginning of formulating a recipe is important because of the possibility of reaching a certain level, which is by definition recommended for various latex paints with generally defined parameters. It is known that in various technical materials and starter formulations, very often PVC levels are determined from which one can expect the desired test results, e.g. wet scrub resistance, coating gloss, water vapor permeability (porosity), etc.

Using such tables recommendation for individual PVC levels is extremely important, however, with the proviso that each test of a specific parameter, e.g. resistance to wet scrubbing, can be enhanced by the use of raw materials that increase strength, e.g. nepheline syenite in paints with slightly higher PVC to improve scrub resistance. Starting the formulation of the paint from a specific PVC range allows you to start with further tests already in a certain range that can be adjusted plus or minus in the range, e.g. PVC 60% ± 10%. It should always be remembered that formulating a paint based on PVC can not only consist in calculating the recipe in MS Excel or software for formulating, but must go hand in hand with the subsequent preparation of samples and laboratory tests, because after the conversion of PVC and the relationship with with other formulation solids, the exact parameters of paints and coatings cannot be determined and sample preparation and testing are required to verify the performance of the raw materials.

The table below (Fig. 4) shows the basic principles of building a formulation based on PVC, relating them to typical latex paint formulations for the EU market [3]. Of course, these ranges are only a starting point that requires adjustment depending on the raw materials used (mainly fillers, their density, type, share of functional fillers, but also grades of titanium dioxide used, their density, etc.). These formulations relate to general ranges based on 50% solids latex emulsions, however for other solid contents these formulations also need to be modified. In this case, it is very helpful to formulate recipes based on the determination of PVC, because PVC does not depend on binder solid content, because we refer to the volume of the dry film-forming substance (binder).

Fig. 4 General rules for building latex paint formulations for the EU market with the use of PVC

The selection of PVC is key to determining the most desirable range of dirt pick-up resistance (DPUR), hiding power or scrub resistance. Fig. 5 shows an exemplary plot of the scrubbability relationship of paints based on two different styrene-acrylic emulsions, in which PVC was tested from 30% to 80% for the desired level of scrubbability. As you can see, the differences between the latex emulsions used are clearly visible in cycles to failure, especially in the range of PVC from 30% to 70%, which indicates that it is not possible to clearly indicate on the basis of PVC what will be the resistance to scrubbing, but only the expected range, because the final the result depends on the raw materials used, in this case the latex emulsion.

Fig. 5 Case study test results – dependence of scrub resistance on PVC for paints based on two different latex emulsions

Of course, the characteristics of the formulation in terms of the dependence of PVC on scrubbability is not the only way to express the influence of PVC on the parameters of coatings. The influence of PVC on water vapor permeability is often characterized, while examining the influence of the formulation ingredients on obtaining the highest or the lowest possible water vapor permeability (also carbon dioxide permeability), PVC dependence on gloss, dirt pick-up resistance, etc. In the Spektrochem laboratory, we carry out such extensive case studies as standard in order to characterize the impact of raw materials and PVC with their participation on various parameters of coatings.

Volume as basic formulation constant

Following PVC as a formulation solid, expressed by volume, or the volumetric packaging system of paints (liter, gallon), in latex paints formulation, volume solids are also expressed, which determines the volumetric content of non-volatile parts in the liquid paint. The value of volatile substances by volume can be calculated from the recipe, however, the value determined on the basis of laboratory determination is used for technical data sheets and other calculations in paint tests.

The term volume solids refers to liquid paint, however it is practically denoted by weighing the dry coatings applied by dipping on standardized discs. Weighing is performed in air as well as in water, and using the laws of hydrostatic buoyancy and the non-volatile weight and density data, the volume solids data obtained are calculated in% by volume, and the test is performed in accordance with ASTM D2697 (Fig. 6).

Fig. 6 Coating of latex paint during water weighing on disk for volume solids determination (ASTM D2697)

In a simple quality control of latex paints, the solid content by weight determination is performed, e.g. using a moisture analyzer, obtaining a result of e.g. 52 wt%, which means that the paint consists of 52% by weight of non-volatile parts. The volatile content by volume, practically determined in accordance with ASTM D2697, refers not to the weight content, but the volumetric content of the components that form the coating after the liquid paint dries. With standard formulations based on carbonate fillers as well as functional but not lightweight fillers, the volume remaining after all the components of the latex paint have evaporated are usually much lower than the weight value. Standard formulations with a non-volatile content of 52% by weight usually contain from 25 to 30% volume solids.

This means that from the coating which was applied in a 100 µm wet layer, about 25 to 30 µm dry coating remained after drying. In practice, this means that 70 to 75% of the volume of the wet film has evaporated. Therefore, the standard in high-quality paints is a minimum of 40% volume solids, which can be mainly found on the US and Canada [4] markets, however, the practice of formulating latex paint recipes is also becoming popular in other regions of the world, including the EU. Obtaining volume solids at a minimum level of 40% is possible when formulating low-to-medium PVC paints, because due to the weight of fillers used as standard in high PVC paint formulations, it does not allow for such a high content of volume solids. In very high-quality paints, the so-called in one-coating paints, this content should be from 42 to even 50% volume solids.

Thanks to such formulation of recipes, not only more paint is left on the substrate, but also the content of titanium dioxide in the formulations of the PVC medium can be reduced due to the larger volume of the coating, which provides coverage. Recipe calculations in the respective software are helpful here, however, they must always be supported by the determination of the volume solids using the ASTM D2697 test. In paints where it is not possible to obtain a high volume remaining after the coating dries, the solution is light fillers, the share of which can increase volume solids even in the initial high PVC.

Summary

In my opinion, building latex paint recipes by building the core to achieve a specific PVC and PVC and CPVC dependencies is the best way to formulate recipes. Building recipes on the basis of adding certain raw materials to the recipe is not a good way because the formulation takes much longer in this way. It requires a series of samples, testing and reformulation to find the appropriate coating parameters. Building a formulation based on PVC allows you to build parameters in a specific area of expectations, especially when using raw material guidelines and selectors with a specific influence of individual raw materials and doses on PVC.

References

[1] ISO/DIS 4618 Paints and varnishes – Vocabulary (voting draft from 2022-03-14 to 2022-06-06)
[2] Getting the ratio right, Artur Palasz, European Coatings Journal, September 2022
[3] Solvay Typical Formulations of Emulsion Paints (GA-BBm-1103)
[4] Volume solids in paints. Why is it considered very important almost only on the NAFTA paint market?