Effect of Dispersing Agents on the Stability of Recycled Paints

Abstract

In a circular economy approach, paint waste is recycled and reformulated to be transformed into new paints. In this regard, the lifespan of recycled paints must be properly assessed and extended beyond their preliminary specifications. This study aimed to improve the stability of waterborne acrylic paints by adding dispersing agents. Seventeen dispersing agents were added to two formulations of recycled paints: one stable, and one showing signs of instability. Their stability was characterized by analytical centrifugation and quantification of the supernatant. Among the tested dispersing agents, four showed a significant improvement in stability in both tests. These dispersants belong to the four possible categories of stabilization mechanisms: electrostatic, steric, electrosteric, and controlled stabilization. Electrosteric dispersing agents are especially effective in both paints. The combination of two dispersants does not have a synergistic effect.

1. Introduction

Facing the climate and environmental crisis, it is necessary to move from a linear to a circular economy model where waste becomes a resource. This circular economy can be applied within the paint industry by recycling waste from unused paints to transform it back into new paints [1,2]. This innovation has become a reality for waterborne acrylic paints. However, the original formulation of these products does not take into account their recycling and second life. It is thus important to study the mechanisms of stability of these paints. This understanding will allow for improved paint stability.

Different types of destabilizations can occur in dispersion, with simultaneous mechanisms in some cases. These destabilizations are physical phenomena involving a modification of the structural organization of the dispersion. The loss of stability and the associated mechanism not only depend on the intrinsic characteristics of the dispersion but also the environmental conditions such as the temperature. Some mechanisms are reversible while others are permanent. The destabilization phenomena that can occur in paints are sedimentation, flocculation, and/or syneresis. They are all reversible [3,4,5].

Paint is composed of five main families of compounds: a binder, providing mechanical properties; a solvent, which reduces the viscosity of the medium; pigments for color and opacity; fillers that add volume as well as certain physical–chemical properties; and additives that adjust specific properties [6]. The phenomena of destabilization mentioned for paint will concern and involve the solid particles present, namely the pigments and the fillers.

These destabilization phenomena are influenced by several factors such as the size and volume fraction of the dispersed particles, the viscosity of the continuous phase, and the type and concentration of dispersing agents [7,8]. By modifying these parameters, the destabilization phenomena can be limited. In the case of recycled paints based on already formulated products, the actions used in order to increase stability are more restricted. Indeed, the particle size and the volume fraction cannot be modified, and the viscosity must remain within a certain range. The solution is made by adding post-addition dispersing agents to increase the stability.

Dispersing agents adsorb on the particle interface and create a membrane around the particles which prevents them from aggregating. Figure 1 shows the three categories of dispersants—polymers, polyelectrolytes, and fatty acid derivatives—and the four types of possible stabilizations: steric, electrostatic, electrosteric, and controlled flocculation [9,10,11]. Steric stabilization is based on steric hindrance between the particles due to the unfolding of the polymer’s side chains in the continuous phase. Electrostatic dispersing agents include ionizable groups, such as polyacrylate, that create electrostatic repulsion between the particles. Electrosteric stabilization combines the two previous mechanisms using a polyelectrolyte with sufficiently long chains for steric stabilization [12]. Dispersing agents based on controlled flocculation interact with each other via hydrogen bonds after adsorption on the particles. The particles thus bound form a three-dimensional network, which slows down the sedimentation [13]. The chemical structure of the dispersant as well as the anchoring groups present will impact its efficiency.

The objective of this study is to evaluate the stability of recycled paints and study the effects of different post-addition dispersing agents in various amounts. Several analytical methods are available to characterize the stability of paint dispersion, such as rheological measurements or microscopy analyses [14,15]. Among them, two methods were selected: the measurement of the instability index after acceleration of the destabilization by analytical centrifugation, and the evaluation of the supernatant released after several weeks of storage at ambient temperature. The first method provides a fast quantification of the stability with no necessity for dilution or the identification of the destabilization phenomena involved [16]. The second one evaluates the stability of the paint in real conditions, with the supernatant destabilization resulting from syneresis [17].

2. Materials and Methods

2.1. Materials and Samples Preparation

2.1.1. Recycled Paints

Two formulations based on recycled paints were manufactured for this study, with a similar process to the paints produced by CIRCOULEUR (Blanquefort, France). The first formulation was a stable white matte paint designated as a stable recycled paint (SRP) and the second one was a gray matte paint presenting signs of instability and designated as unstable recycled paint (URP). Both formulations were waterborne acrylic paints.

2.1.2. Dispersing Agents

Seventeen dispersing agents with four different types of stabilization mechanisms were tested: electrostatic, steric, electrosteric, and controlled flocculation. The chemical natures of dispersing agents are detailed in

Table 1. Chemical nature of the electrostatic dispersing agents.

Electrostatic Dispersing Agents	Chemical Structures	Pigment-Affinic Groups
E1	Sodium polyacrylate	No
E2	Ammonium polyacrylate	No
E3	Copolymer of potassium acrylate and acrylic ester	No
E4	Hydrophobically modified potassium polyacrylate	Yes

,

Table 2. Chemical nature of the steric dispersing agents.

Steric Dispersing Agents	Chemical Structures	Pigment-Affinic Groups
S1	Polyether functionalized by phosphoric acid	Yes
S2	Copolymer based on polyether	Yes
S3	Copolymer based on polyether	Yes
S4	Copolymer based on polyether	Yes
S5	Phenol ethoxylate	Yes
S6	Alcohol ethoxylate	Yes
S7	Nonionic polymer	No
S8	Copolymer based on polyether	Yes

,

Table 3. Chemical nature of the electrosteric dispersing agents.

Electrosteric Dispersing Agents	Chemical Structures	Pigment-Affinic Groups
ES1	Polyfunctional acrylate copolymer	Yes
ES2	Acrylate copolymer	Yes
ES3	Acrylate copolymer	Yes
ES4	Acrylate copolymer	Yes

and

Table 4. Chemical nature of the controlled flocculation-based dispersing agent.

Controlled Flocculation Dispersing Agent	Chemical Structure	Pigment-Affinic Group
FC	Fatty acid derivative	Yes

.

2.1.3. Addition of Dispersing Agents in Paints

The selected dispersing agents were added at 0.2 or 1% of active matter in 100 g of SRP or URP recycled paint. The paint was stirred at 1100 rpm with a high-speed disperser (Dispermill Orangeline, Erichsen, Valence, France) fitted with a 40 mm blade. Dispersing agents were incorporated by double weighing and stirred for 5 min.

2.2. Measurement of the Instability Index

The stability of the paints was evaluated by analytical centrifugation using a commercial tool (LUMisizer^®, LUM GmbH, Berlin, Germany) with STEP technology^® (Space and Time resolved Extinction Profiles). Destabilization is accelerated by centrifugation and the entire sample is illuminated by passing it in front of a light source. The amount of light transmitted is detected by multiple detectors with micrometer resolution and converted to extinction. By monitoring the extinction profiles of the entire sample over time, the type of destabilization (sedimentation, creaming, or syneresis/flocculation) can be identified [18,19]. The stability of a product can be quantified with the instability index from those profiles. The instability index is the clarification (namely the transmission increase) at a given separation time divided by the maximum clarification [20].

The samples were collected with a 2 mL syringe and introduced with a cannula into 2 mm optical path polyamide analysis cells (triplicates of the experiment). The samples were illuminated by a near-infrared source at 865 nm. The light intensity factor was set to 6 due to the opacity of the samples. The analysis was performed at 25 °C and an acceleration of 300× g (1600 rpm). The total analysis lasted 6 h with the following cycle of measurements:

• 10 measurements with an interval of 10 s between each measurement;
• 10 measurements with an interval of 30 s between each measurement;
• 101 measurements with an interval of 210 s between each measurement.

The instability indexes obtained were plotted as a function of time. The values of the instability indexes at 6 h (end of the analysis) were determined and averaged on the triplicates.

2.3. Evaluation of Supernatant Height

The samples were collected with a 10 mL syringe and introduced with a cannula into 15 mL Falcon tubes which were placed in an oven at 25 °C. The height of the supernatant was evaluated after 10 weeks.

3. Results

3.1. Instability Index

3.1.1. Stable Recycled Paint with Dispersing Agents

The transmission profiles of the Stable Recycled Paint (SRP) are shown in Figure A1 in Appendix A and enable the identification of syneresis/flocculation and sedimentation phenomena [21,22].

Figure 2 shows the instability index values after 6 h of analysis at 300× g in SRP with 0.2 or 1% of dispersing agents. The lower the instability index, the more stable the paint is. The dispersing agents added can be classified according to how effectively they increase the stability of the formula.

Dispersing agents with an electrostatic mechanism showed a low impact on the instability index at 0.2% of addition, except for E4 which exhibited a lower value than the reference (0.27 and 0.34, respectively). The SRP paint is therefore more stable with 0.2% of the dispersing agent E4. At 1% of addition, the electrostatic dispersing agents cause an increase in the instability index, leading to a negative impact on the stability of the paint. Only the steric dispersing agents S5 and S6 are effective at 0.2% with a reduction in the instability index. The paint with the dispersing agent S6 at 1% of addition also presented a lower value than the reference, as well as the paint with 1% of S1. The other dispersing agents at 1% induced a higher instability index. The dispersing agent S4 produced the highest instability index, with a value of 0.48 at 1% in SRP. The electrosteric dispersing agents are the most effective, except for ES3. ES2 and ES4 only reduced the instability index at 1% of addition, while ES1 caused a decrease with 0.2 and 1% of addition. The lowest value of the instability index was reached with 1% of the dispersing agent ES1: 0.21 versus 0.34 for the reference. The dispersing agent CF, based on controlled flocculation, seemed to degrade the stability, with a higher instability index at 0.2 and 1% of addition.

3.1.2. Unstable Recycled Paint with Dispersing Agents

As shown in Figure A2 in Appendix A, the unstable recycled paint (URP) exhibited the same type of transmission profiles as SRP, with syneresis/flocculation and sedimentation phenomena [21,22]. Figure 3 shows the values of the instability index for URP with 0.2 or 1% of dispersing agents after 6 h of analysis at 300× g. As for SRP, only E4 provides a decrease in the instability index among the electrostatic dispersing agents. However, this reduction from 0.29 to 0.10 is obtained with a 1% of addition and not 0.2%. None of the steric dispersing agents were effective in URP, except S6 added at 1% which exhibited a lower instability index than the reference. For all, except S1, the instability index was higher with 0.2% of dispersing agents than with 1%. The electrosteric dispersing agents ES1 and ES2 presented a very low instability index at 1% of addition, 0.15 and 0.17, respectively. The values were higher than the reference with the four electrosteric dispersing agents added at 0.2%. The samples with the dispersing agent FC at 0.2 and 1% displayed an increase in the instability index compared to the reference.

3.1.3. Combining Dispersing Agents

According to the previous tests, particular dispersing agents can reduce the instability index and improve the stability of recycled paints. Further testing was performed to determine whether the combination of two dispersing agents with different stabilization mechanisms provided a synergic effect. The dispersing agents selected were E4 (electrostatic), S6 (steric), and ES1 (electrosteric) because of their positive impact on the stability of both paints.

Figure 4 and Figure 5 show the instability index of different combinations of those three dispersing agents in SRP and URP, respectively. The values are compared to those obtained with the dispersing agents added individually at 0.2 or 1% in the paint. For both paints, the instability index was between the individual values of the combined dispersing agents, or even higher. In URP, the sample with 0.5% of ES1 and 0.5% of E4 presented an instability index of 0.21 while the values with individual dispersing agents were 0.15 and 0.10, respectively. Increasing the addition level of the combined dispersing agents only increased the instability index. The order in which the dispersing agents were incorporated did not impact the instability index. The samples of URP with 1% of ES1 and 1% of E4 both presented an instability index of 0.25, regardless of the sequence of incorporation.

3.2. Evaluation of the Supernatant at 25 °C

To validate the results obtained with the LUMisizer in accelerated conditions, an evaluation of supernatant release was performed over a period of 10 weeks at 25 °C. This period of 10 weeks in real conditions (namely at 1× g) corresponds approximately to the accelerated conditions of 6 h at 300× g [23]. The formation of the supernatant at the surface of the paint is due to the destabilization phenomenon of syneresis. The higher the amount of supernatant, the stronger the syneresis [24].

3.2.1. Stable Recycled Paint with Dispersing Agents

Figure 6 shows the values of the supernatant height at 10 weeks in SRP with 0.2 or 1% of dispersing agents. At 1% of addition, the electrostatic dispersing agents E1, E2, and E3 exhibited a quantity of supernatant nearly three times higher than the reference. The supernatant height was also higher with 1% of E4 than with 0.2%, but both samples released less supernatant than the reference. Concerning the steric dispersing agents, S1, S3, S5, and S6 reduced the supernatant height at 0.2 and 1% of addition, as well as S8 at 0.2%. The other steric dispersing agents led to an increase in the quantity of supernatant, especially S7 added at 1%. The electrosteric agents had a strong reducing effect on the supernatant released, with only 0.1 or 0.2 graduations compared to 0.5 graduations for the reference. The dispersing agent CF, based on controlled flocculation, revealed no effect at 1% of addition but helped reduce the supernatant height when added at 0.2%.

3.2.2. Unstable Recycled Paint with Dispersing Agents

Figure 7 shows the values of the supernatant height at 10 weeks in URP with 0.2 or 1% of dispersing agents.

The addition of the electrostatic dispersing agents increased the amount of supernatant, except for E2 and E4 at 1%. The steric dispersing agents were quite efficient and reduced the supernatant height with only S2, S4, and S5 at 1% increasing it. As for SRP, all of the electrosteric dispersing agents resulted in a reduction in the quantity of supernatant at both addition levels. However, the reduction was stronger at 1% of addition, as well as for the CF dispersing agent. The amount of supernatant in the reference URP was higher than in SRP (0.9 versus 0.5 graduations), but the maximum decrease brought by the dispersing agents was quite similar: 0.2 graduations in URP and 0.1 in SRP.

3.3. Correlations between Instability Index and Supernatant Height

The results of the instability index (accelerated conditions) and the supernatant height (real conditions) were collated on a graph to establish correlations between them. The concordance of the results has been highlighted by a green or red area in case of a positive or negative impact on the stability, respectively.

3.3.1. Correlations in Stable Recycled Paint

Figure 8 and Figure 9 show the correlations between the instability index and supernatant height in SRP with 0.2 and 1% of dispersing agents, respectively.

The results were quite consistent, with only a few samples out of the green or red areas. The out-of-range samples were the samples with 0.2% of S1, S3, S8, ES2, ES4, or CF and with 1% of E4 or S5. All of these inconsistencies gave a higher instability index result (or a lower amount of supernatant) than expected. Therefore, those dispersing agents presented a negative impact on stability according to the instability index, and a positive impact according to the supernatant height. The most promising products appeared to be the electrosteric ones at 1% of addition, especially ES1, which was also efficient at 0.2%, or the electrostatic dispersing agent E4 added at 0.2%. Electrosteric dispersing agents were expected to be efficient because of the double stabilization mechanism.

3.3.2. Correlations in Unstable Recycled Paint

Figure 10 and Figure 11 show the correlations between the instability index and supernatant height in URP with 0.2 and 1% of dispersing agents, respectively. Unlike SRP, the results between the two tests were not consistent in URP at both levels of addition. No correlation can be established for the positive effect on stability when adding 0.2% of dispersing agents. The electrostatic dispersing agents E1 and E3 worsened the stability when added at 0.2 or 1%. The electrosteric dispersing agents ES1, ES2, and ES4 and the steric agent S6 were effective at 1% of addition, as well as in SRP. The electrostatic dispersing agent E4 also gave very good results when added at 1%.

4. Discussion

As shown, the stability of recycled paints can be improved with certain dispersing agents. For URP, adding 0.2% of dispersing agents was insufficient to enhance the stability according to the instability index.

Most electrostatic dispersing agents were ineffective and even worsened the stability according to both tests, except for E4. This hydrophobically modified polyacrylate includes pigment-affinic groups, probably aromatic rings, which enable proper anchoring on the particles. The other electrostatic dispersing agents do not contain this kind of group, which may explain the difference in efficiency. However, the dispersing agent E4 had a negative impact on the stability at a 1% addition in SRP. With this higher amount, not all of the dispersing agent molecules may have been adsorbed due to a saturation of the particle surface. Non-adsorbed dispersing agents remain in the continuous phase leading to depletion flocculation [25,26]. This phenomenon could explain the increase in the instability index with 1% of E4 in SRP.

Steric dispersing agents present little efficiency according to the instability index. Only the dispersing agent S6 reduced the instability index in both paints. The chemical structure may explain its efficiency compared to other dispersants. With the ethylene oxide derivatives responsible for steric repulsion being on secondary chains, they can better unfold around the particles. This dispersing agent could have different kinds of pigment-affinic groups that could help the anchoring of the particles. Considering the supernatant height, the dispersing agents S1, S3 and S5 could improve the stability of both paints. Those results did not correlate well with the instability index, similarly, the results with S7 and S8 in the URP paint. Several hypotheses can explain this lack of correlation. Stability under real conditions is evaluated with the supernatant height at 25 °C. However, this parameter only reflects syneresis destabilization and does not consider flocculation or sedimentation as in the case of the instability index. The overall destabilization of the products can therefore be underestimated. On the other hand, centrifugal acceleration for the instability index can amplify destabilization or create new mechanisms. The prolonged contact of the particles due to centrifugation can, for example, induce a flocculation phenomenon. This flocculation might not have occurred under real conditions.

Electrosteric dispersing agents improve the stability of both paints, especially at 1% of addition. ES1 is the most effective, while the efficiency of ES3 is quite low. These two dispersing agents present the same chemical structure but most likely have different chain lengths and/or arrangements, which would explain the contrast in efficiency.

Results of the instability index and supernatant height are not correlated for the dispersing agent based on controlled flocculation in both paints. This fatty acid derivative is supposed to improve stability thanks to the hydrogen bonds between the molecules that create a three-dimensional network. The higher instability index might be explained by the fragility of the three-dimensional network which would not resist centrifugation during the analysis. Considering the supernatant height results, this dispersing agent helps improve the stability of both paints.

No synergistic effects were found when adding two dispersing agents with different mechanisms. Dispersing agents appear to be more efficient individually. The combination of two dispersing agents does not provide a synergistic effect but rather an antagonistic effect. As the tests were operated at the same rate of active ingredient, the surface saturation of the particles is not an issue. The competitive adsorption of the dispersing agents might however explain the lack of synergy [27]. One of the dispersing agents adsorbs faster on the particles and hinders the adsorption of the other. Non-adsorbed dispersing agents remain in the continuous phase and may cause depletion flocculation [25,26]. The instability index is higher when combining dispersing agents, thus the stability is lower. These results support the previous hypothesis on competitive adsorption.

The most promising dispersing agents were E4 (electrostatic), S6 (steric), and ES1 and ES2 (electrosteric) at 1% of addition. The addition rate needs to be adapted for E4, as it is effective only at 0.2% in SRP, and only at 1% in URP. The dispersing agent CF based on controlled flocculation should also be considered, regarding the results with supernatant height.

Dispersing agents can also impact other properties of the paint, such as color, opacity, gloss, rheological behavior, adhesion, and wet abrasion resistance [11,12]. Indeed, these properties depend in part on the size and/or shape of the particles in the paint, which can be modified by the incorporation of dispersing agents. All of these properties will therefore be checked to ensure that the dispersing agents do not modify them.