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Coatings 2022, 12, 1722. https://doi.org/10.3390/coatings12111722 www.mdpi.com/journal/coatings
Article
Effect of Dispersing Agents on the Stability of Recycled Paints
Jessie Jacob
1,2,3,
*, Stéphane Grelier
3,
*, Maïlys Grau
1
and Blandine Chorein
2
1
CIRCOULEUR, 33290 Blanquefort, France
2
Institut Textile Et CHimique de Lyon, 69134 Ecully, France
3
Laboratoire de Chimie des Polymères Organiques, 33607 Pessac, France
* Correspondence: jjacob@circouleur.fr (J.J.); sgrelier@enscbp.fr (S.G.)
Abstract: In a circular economy approach, paint waste is recycled and reformulated to be trans-
formed 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.
Keywords: stability; recycling; acrylic waterborne paint; dispersing agents; analytical
centrifugation; supernatant height
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 mech-
anisms in some cases. These destabilizations are physical phenomena involving a modi-
fication of the structural organization of the dispersion. The loss of stability and the asso-
ciated mechanism not only depend on the intrinsic characteristics of the dispersion but
also the environmental conditions such as the temperature. Some mechanisms are reversi-
ble while others are permanent. The destabilization phenomena that can occur in paints
are sedimentation, flocculation, and/or syneresis. They are all reversible [3–5].
Paint is composed of five main families of compounds: a binder, providing mechan-
ical 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.
Citation: Jacob, J.; Grau, M.;
Chorein, B.; Grelier, S. Effect of
Dispersing Agents on the Stability of
Recycled Paints. Coatings 2022, 12,
1722. https://doi.org/10.3390/
coatings12111722
Academic Editor: Ivan Jerman
Received: 12 October 2022
Accepted: 4 November 2022
Published: 11 November 2022
Publisher’s Note: MDPI stays neu-
tral with regard to jurisdictional
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tional affiliations.
Copyright: © 2022 by the authors. Li-
censee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and con-
ditions of the Creative Commons At-
tribution (CC BY) license (https://cre-
ativecommons.org/licenses/by/4.0/).
Coatings 2022, 12, 1722 2 of 13
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 dis-
persing 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–
11]. Steric stabilization is based on steric hindrance between the particles due to the un-
folding 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.
Figure 1. Stabilization mechanisms according to the type of dispersant.
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 ana-
lytical 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 phenom-
ena 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.
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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 Tables 1–4.
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 co-
polymer Yes
ES2 Acrylate copolymer Yes
ES3 Acrylate copolymer Yes
ES4 Acrylate copolymer Yes
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 com-
mercial tool (LUMisizer®, LUM GmbH, Berlin, Germany) with STEP technology® (Space
Coatings 2022, 12, 1722 4 of 13
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 con-
verted 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 in-
crease) 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 accelera-
tion of 300× g (1600 rpm). The total analysis lasted 6 h with the following cycle of meas-
urements:
• 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 trip-
licates.
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 in-
crease the stability of the formula.
Figure 2. Instability index of SRP with 0.2 or 1% of dispersing agents (6 h at 300× g). Refer to Tables
1–4 for dispersing agent labels. Error bars depict sampling error. Values are significantly different
from the control SRP (p < 0.05) except for nonsignificant results (ns).
Coatings 2022, 12, 1722 5 of 13
Dispersing agents with an electrostatic mechanism showed a low impact on the in-
stability 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 sedimenta-
tion 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, re-
spectively. The values were higher than the reference with the four electrosteric dispers-
ing agents added at 0.2%. The samples with the dispersing agent FC at 0.2 and 1% dis-
played an increase in the instability index compared to the reference.
Figure 3. Instability index of URP with 0.2 or 1% of dispersing agents (6 h at 300× g). Refer to Tables
1–4 for dispersing agent labels. Error bars depict sampling error. Values are significantly different
from the control URP (p < 0.001) except for nonsignificant results (ns).
Coatings 2022, 12, 1722 6 of 13
3.1.3. Combining Dispersing Agents
According to the previous tests, particular dispersing agents can reduce the instabil-
ity 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 (electro-
static), S6 (steric), and ES1 (electrosteric) because of their positive impact on the stability
of both paints.
Figures 4 and 5 show the instability index of different combinations of those three
dispersing agents in SRP and URP, respectively. The values are compared to those ob-
tained 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 incorpo-
rated 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.
Figure 4. Instability index of SRP with different combinations of dispersing agents and comparison
with the dispersing agents added individually (6 h at 300× g). Refer to Tables 1–4 for dispersing
agent labels. Error bars depict sampling error. Values are significantly different from the control
SRP (p < 0.01) except for the nonsignificant result (ns).
Figure 5. Instability index of URP with different combinations of dispersing agents and comparison
with the dispersing agents added individually (6 h at 300× g). Refer to Tables 1–4 for dispersing
agent labels. Error bars depict sampling error. Values are significantly different from the control
URP (p < 0.0001).
Coatings 2022, 12, 1722 7 of 13
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 re-
leased 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, espe-
cially S7 added at 1%. The electrosteric agents had a strong reducing effect on the super-
natant released, with only 0.1 or 0.2 graduations compared to 0.5 graduations for the ref-
erence. 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%.
Figure 6. Supernatant height of SRP with 0.2 or 1% of dispersing agents (10 weeks at 25 °C). Refer
to Tables 1–4 for dispersing agent labels.
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.
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Figure 7. Supernatant height of URP with 0.2 or 1% of dispersing agents (10 weeks at 25 °C). Refer
to Tables 1–4 for dispersing agent labels.
The addition of the electrostatic dispersing agents increased the amount of superna-
tant, except for E2 and E4 at 1%. The steric dispersing agents were quite efficient and re-
duced 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 dis-
persing 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
Figures 8 and 9 show the correlations between the instability index and supernatant
height in SRP with 0.2 and 1% of dispersing agents, respectively.
Figure 8. Correlation between the instability index and supernatant height in SRP with 0.2% of dis-
persing agents. Refer to Tables 1–4 for dispersing agent labels. Green area = positive impact on sta-
bility, red area = negative impact on stability.
Coatings 2022, 12, 1722 9 of 13
Figure 9. Correlation between the instability index and supernatant height in SRP with 1% of dis-
persing agents. Refer to Tables 1–4 for dispersing agent labels. Green area = positive impact on
stability, red area = negative impact on stability.
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 pre-
sented a negative impact on stability according to the instability index, and a positive im-
pact 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
Figures 10 and 11 show the correlations between the instability index and superna-
tant 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 dis-
persing 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%.
Figure 10. Correlation between the instability index and supernatant height in URP with 0.2% of
dispersing agents. Refer to Tables 1–4 for dispersing agent labels. Green area = positive impact on
stability, red area = negative impact on stability.
Coatings 2022, 12, 1722 10 of 13
Figure 11. Correlation between the instability index and supernatant height in URP with 1% of dis-
persing agents. Refer to Tables 1–4 for dispersing agent labels. Green area = positive impact on sta-
bility, red area = negative impact on stability.
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 in-
cludes 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 lead-
ing 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 ox-
ide 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. Stabil-
ity 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 pro-
longed 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 dis-
persing agent based on controlled flocculation in both paints. This fatty acid derivative is
Coatings 2022, 12, 1722 11 of 13
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 dur-
ing 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 satura-
tion 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 there-
fore be checked to ensure that the dispersing agents do not modify them.
Author Contributions: Conceptualization, J.J.; methodology, J.J.; formal analysis, J.J.; writing—orig-
inal draft preparation, J.J.; writing—review and editing, J.J., M.G, B.C. and S.G.; supervision, M.G.,
B.C. and S.G. All authors have read and agreed to the published version of the manuscript.
Funding: This research was financially supported by CIRCOULEUR and ANRT (2018/1768).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Conflicts of Interest: The authors declare no conflict of interest.
Appendix A
The transmission profiles of the stable recycled paint (SRP) and the unstable recycled
paint (URP) are shown in Error! Reference source not found. and Error! Reference source
not found. respectively, respectively. Each curve represents a profile at a time t of the
analysis, with the first profile in red and the last in green.
Coatings 2022, 12, 1722 12 of 13
Figure A1. Transmission profiles of the stable recycled paint, SRP (6 h at 300× g).
Figure A2. Transmission profiles of the unstable recycled paint, URP (6 h at 300× g).
Profile changes are due to the variation in particle concentration in the sample and
also to changes in the size or shape of the particles. The parallel lines at the top of the
graph indicate syneresis and/or flocculation [18]. The supernatant appears gradually at
the top of the tube leading to total light transmission. The transmission gradient at the
bottom of the spectrum is due to the sedimentation of polydisperse particles. The particles
do not move at the same velocity depending on their size, with a decrease at high concen-
trations because of the hindrance [28].
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