Abrasion Resistance of Paints Made with Curcumin Pigment and PVA Resin

Artisanal paints made with natural ingredients are composed of pigments, solvents, binder, and additives. The function of the pigment is to give color to the paint mixture, the solvent keeps the pigments in a homogeneous configuration and improves the applicability of the paint mixture, evaporating after application, while the additives are used to optimize the characteristics and durability of the paints. The binder is the non-volatile part of the paint, responsible for uniting the pigment particles, forming an adhesive film when applying the paint to the surface [1, 2].

The binder is used in the manufacture of paints since cave paintings dated between the years 30.000 to 8.000 BC, in which the inhabitants of caves felt the need to use local protein inputs as a “glue” to adhere the pigments to the walls, such as eggs (albumin protein), resins and vegetable oils [3, 4, 5].

Over time, other possibilities for binders were discovered, such as natural resins extracted from acacia species (popularly known as gum arabic), gelatin (collagen extracted from the skin, tendons and bones of pigs, cows and chickens, and from the skin and scales of fish), cow’s milk (casein protein) and beeswax [6, 7]. In South America and Africa, it was common to use latex (resin extracted from Hevea euphorbiacex) as a binder and for waterproofing surfaces, while in Mexico and Colombia it was common to use resin extracted from the “nopal” cactus (Nopalea coccinel-lifera, Cactus cochenilliferus), known as “fig tree”, for the same purposes [7]. Other vegetable resins commonly used in the elaboration of natural dyes are rye flour glue with the addition of zinc sulfate (ZnSO₄), agave sap (Agave tequilana), juice of cooked banana leaves, sap of Cactus opuntia, sap of Euphorbia lactea, kapok oil (Chorisia speciosa) and cellulose glue [8].

Despite being a product of industrial origin, the use of polyvinyl acetate resin (PVA) as a binder (popularly known as “white glue”) is recommended in the production of building paints due to the lack of toxicity, high solubility in water, and being classified as the best alternative to glues by the German government [9], in addition to the absence emissions of volatile organic compounds (VOCs). According to the database of the University of Delft, in the Netherlands [10], PVA white glue requires 58 MJ/kg of energy for its manufacture, equivalent to 12.9 MJ/l in recipes for paints with soil pigment 11, that is, a relatively low value compared to 65 MJ/kg of industrialized PVA paints [12]. As the objective of making natural paints is also to encourage self-production, PVA white glue is commonly found on the market, which makes it practical to use as a binder, despite being considered the most expensive component in the making of natural paints.

Currently, it is common to find recipes and experiments published by researchers of building paints area with natural ingredients in which PVA resins are used as a binder, and the colors are obtained through mineral pigments extracted from clays and lime [11, 13]. However, vegetable pigments are little explored in construction area due to their instability in the face of adverse weather conditions.

Therefore, the present study fills this gap through the development of paints formulated through the method of experimental mixture planning, in which the amounts of PVA resin in the mixtures were varied within the limits stipulated by a bibliographic review in order to evaluate its influence as a binder on the characteristics of pH, viscosity and abrasion resistance (RAU) characteristics of artisanal paints made with curcumin pigment extracted from the root of Curcuma Longa L., popularly known as turmeric.

Figure 1 shows the 36 paint mixtures made with curcumin pigment applied to a mortar substrate of 10 cm x 10 cm, according to procedures described in NBR 15380 [25] for the RAU test. From the image it is possible to notice the difference in the shade of the yellow color as the amounts of pigment, solvent, additives vary, and according to the addition of painting lime which, due to its alkalinity, results in a change in the color of the pigment, as occurs with the bixin pigment, extracted from annatto (Bixa orellana) [26].

Figure 1.

The squares with the images of the specimens painted with the paint mixtures are identified with the corresponding mixture number (1 to 36)

Table 3 presents the results of the response variables analyzed at each experimental point, as well as the density (g/cm³) of each mixture and its temperature (°C) at the moment of measuring the cinematic (v) and dynamic viscosity (μ). In the table are highlighted in bold the values considered acceptable for viscosity for good application of the paint on the substrate (minimum of 70 cp and maximum of 160 cp), the pH values within the limits described according to the bibliographic review (between 4.5 and 5.5), and RAU above 100 brushing cycles considered good performance for economic category latex paints.

From the analysis of the response variables, it is possible to identify a pattern when the PVA resin is used at the minimum and maximum levels, presenting lower RAU and viscosity when the amount of PVA resin reaches the maximum in the paint emulsion, regardless of the pH value. The minimum amount of PVA resin in the paint emulsion reached sufficient RAU when the viscosity was high, regardless of the pH value, while the minimum amount of resin resulted in insufficient RAU when the viscosity and pH values of the emulsions were lower to the stipulated limits. Furthermore, sufficient RAU was achieved when the emulsions had low viscosity, regardless of the pH value.

According to the results of the analysis of the response variables, 30% of the prepared mixtures had a viscosity between 70 and 160 cp, 19% of the mixtures had a pH between 4.5 and 5.5, and 72% of the mixtures had a RAU ≤ 100 brushing cycles.

The regression equations for the response variables viscosity, pH, and RAU are shown in Table 4, where the components are identified in the equations: pigment (X), solvent (Y), PVA resin (Z), color preservative additive (W) and bactericide (S), considering significant interactions (p-value < 0.05).

The regression analysis for the viscosity index indicates that, to obtain viscosity within the specified limits, higher levels of X and S are necessary, and lower levels of Y, Z and W. However, the X factor interacts significantly with the others components, indirectly producing a negative effect. In the same way, component S interacts indirectly with components X, Y and Z, producing a negative effect. Therefore, it is recommended to adopt intermediate and complementary levels for components X and S. It is important to highlight that components X and S are the only solid components of the mixture and, for this reason, they directly influence viscosity levels, since the increase in liquid components Y, Z and W results in a mixture with high viscosity, outside the values considered acceptable for viscosity for good application of the paint on the substrate (minimum of 70 cp and maximum of 160 cp). Therefore, to reduce or completely eliminate the S component while maintaining the levels of the X component, it would be necessary to significantly increase the levels of the Z component in order to maintain the viscosity levels within the values considered acceptable.

The regression analysis for pH indicates that, to obtain higher pH values, it is necessary to increase the levels of factor S and reduce the levels of factors X, Y, Z and W. The S component interacts significantly with the other components, producing a positive effect through this interaction. Therefore, in order to control the effects produced to bring pH values within limits, it is recommended to adopt lower levels of the S component.

The regression analysis for RAU indicates that, to obtain higher resistance values, it is necessary to increase the levels of factors X and S, and reduce the levels of factors Y, Z and W. The S component interacts significantly with W, producing a positive effect indirectly. Therefore, it is recommended to adopt higher levels of factor S to obtain high RAU results.

In order to find the mixture that best represents the joint optimization of the response variables, the present study considered developing a response optimization analysis adopting the same weight for the three variables analyzed using the Minitab 20.4 software, that adjusted the data to simultaneously fit the three response variables through the Desirability method [27], commonly used to determine the best process adjustment conditions, simultaneously optimizing the multiple responses.

The Desirability method found a model fit considered optimal (d = 97%) for each variable, in order to present the ideal composition of each component in the mixture, resulting in a product that meets the requirements performance limits stipulated for an economical latex paint. Therefore, the pH assumed a value of 5.00, the viscosity of 103.89 cp, and the abrasion resistance of 150.49 brushing cycles.

Among the 36 mixtures prepared in the experimental design, only two achieved result within the stipulated limits for the three analyzed variables, the mixtures numbers 2 and 14, which presented adequate viscosity for good applicability of the paint (in the case of paints with vegetable pigment, between 70 and 160 cp), pH between 4.5 and 5.5, and resistance to abrasion above 100 brushing cycles, thus representing the mixtures considered ideal for the elaboration of artisanal paint with Curcuma Longa L. pigment.

It is concluded that the pigment represents a relevant factor not only in fulfilling its common functions in the composition of a paint (providing color and protecting the binder), but is also capable of influencing the viscosity and the ability of the paint to resist brushing. or friction when applied to internal walls. The addition of kakishibu to the mixtures as a colorconserving additive increased the viscosity of the samples and reduced the pH due to its high acidity, generating satisfactory RAU results, since the high pH compromises the action of the PVA resin, according to data collected in a literature review.

Components such as solvent, binder and additive showed unsatisfactory results when used in minimum and maximum proportions, indicating the need to find intermediate proportions to achieve balance in the mixture, achieve appropriate viscosity, abrasion resistance above 100 brushing cycles, and to avoid unnecessary waste with PVA, considered the component with the highest cost in an artisanal paint.

As expected, PVA resin proved to be an effective binder in the RAU tests. However, in the samples to which the bactericide was added, the pH increased, resulting in low RAU, confirming the studies collected in a literature review that stipulated an ideal pH between 4.5 and 5.5 to obtain satisfactory RAU results. For these reasons, based on the calculations and analyzes accomplished through the Minitab 20.4 software, a mixture that fits simultaneously within the specified limits for all variables in the matter is composed of approximately 15% pigment, 20% solvent, 33% PVA resin, 32% additive and does not contain a bactericide.

Besides its main findings, the present study intends to leverage future research in the area of architectural paints made with natural inputs and PVA resin through the initial results of the evaluation of the interaction between this and other components. It is expected to stimulate interest in the possibilities of making architectural paints with other types of chromophores and their interaction with polymeric resins, using as a basis the formulations and characteristics analyzed in the present study.