Type of the Paper Article

Recovery of PS in the Paint or Sealant Industry

Patent WO 2007 072502, in agreement with the principles of the circular economy, pre- sents a process aimed to convert PS into a reusable paint [27]. According to the inventors, several kinds of PS could be suitable for the recycling process, namely amino-alkyd-based, epoxy, acrylic-based, polyester melamine-based, amino polyester-based, thermosetting acrylic, urea-based, silicone or modified silicone-based, and acrylated alkyd PS.
As shown in Figure 3, the process for the production of new paint from PS started with a rinse of the wet sludge with water. PS was subsequently mixed with a solution of
either sodium bicarbonate (NaHCO₃, 3–10% b.w. of the PS), if the booth additive was acidic, or paratoluene sulfonic acid (PTSA, 0.2–0.5%), if the booth additive was alkaline. After a subsequent rinse with methyl alcohol, the residual liquid phase, which wetted the sludge, was removed with pressing, centrifugation, or mild heating. The complete re- moval of the liquid phase was obtained with a drying operation carried out at a tempera- ture range from 35 to 75 °C (or at room temperature) for a period between 1 and 72 h, depending on the sludge condition and quality.

Figure 3. Scheme of the recycling process [27] aimed at converting PS into a reusable paint.

The dried sludge was soaked in one or more solvents (this information was not dis- closed by the patent), depending on the type of PS used as a raw material, and subse- quently stirred in order to obtain a homogeneous mixture. Recommended durations for the two above-mentioned operations were 1–48 h for soaking and 0.5–4 h for stirring. The obtained mixture was subsequently subjected to a series of sequential operations of mill- ing and wet screening (or filtration), as shown in Figure 3. The first and second milling operations were aimed at reducing the filtrate product to sizes of +2 (100 mesh) to +3 (150– 200 mesh) and +6 (300–400 mesh) Hegman’s Gauge fineness, respectively. Particles re- tained in each filtration step were recycled back for reduction to smaller sizes by conven- tional pulverizing equipment. The filtrate product, after filtration at 150–200 mesh, could be added with one or more resins, conventional additives, and pigments, if necessary. Particles with dimensions less than 300–400 mesh were finally suitable to be used as raw materials for the preparation of new paints. The conversion of PS into a reusable paint required several additives, individually or in combination. The choice of the most suitable additive depended on the type of PS. Pigments must also be added in order to achieve the desired color [27].

To the best of our knowledge, the process patented by Bhatia et al. [26] was the only one capable of transforming the waste paint into the same original product or an ingredi- ent of it. However, no applications of such a process were found on an industrial scale. Obstacles to the industrialization of the recycling process were (i) the need to know the exact composition of the paint used in the painting operation, which determines the choice of the reagents necessary for the recovery process; (ii) the inapplicability of the recycling process in the case of a mixed (or contaminated) PS; and (iii) the number of reagents and unit operations required by the recycling process.
Active principles of organic nature contained in PS, namely uncured resins and hy- drocarbons, were deemed of interest for the production of sealants. US Patent N° 4,980,030
protected the first process aimed at recovering PS as a filler in the formulation of sealing products [28]. The painting waste, typically in the form of sludge, was processed with a procedure that included the operations of heating and resin curing. Water and volatile organic compounds (VOCs), in the form of liquid hydrocarbons, were evaporated so that the product, discharged after heating, was in a dried particulate form. The heating opera- tion was also aimed at curing the uncured polymeric paint resins. The inventors recog- nized three main benefits to the patented process:

1. 
   It reduced the final volume of the waste and made its disposal easier and more eco- nomical.
   
2. 
   The power produced can be handled easier than wet sludge.
   
3. 
   It reduced the hazardousness of the waste because liquid hydrocarbons were evapo- rated and toxic metals were bound into the cured resin product.
   

A similar process was reported in US Patent 5,087,375 [29], which disclosed a method to heat and calcine PS in order to produce a powder that could be used as a filler for seal- ants. It has to be underlined that the processes reported in both the afore-mentioned pa- tents [28,29] completely cured any uncured polymers during the heating and/or calcining step. Accordingly, the resulting product was an inert, inorganic, brittle, and abrasive par- ticulate material. An improved process, in which the heating temperature did not exceed 38 °C, was presented in US Patent 5,254,263 [7]. In the presence of such a low temperature, polymer resins remained uncured, and, according to the inventors, the resulting product was soft and easily dispersible as a filler in sealants’ compositions.
However, fillers are not the sole components of sealants, which generally also include other ingredients such as polymers and plasticizers. These components typically account for up to 75% of the overall composition of the sealant, and polymers, in particular, are the principal cost item in the sealant’s formulation. For this reason, it was deemed of in- terest to test the capability of PS to be used as a replacement for one of the polymeric components in sealants in order to reduce production costs.
US Patents 5,880,218 [30], 5,922,834 [31], and 6,455,598 B1 [32] describe a process in which the uncured polymers contained in the treated PS could be used as a partial re- placement of the polymeric components in the formulation of products like heat-curable sealants, pressure-sensitive sealants, caulking sealants, and automotive paintable seam sealers. The process was articulated in the three phases of drying, de-catalysis, and pow- der production through the addition of processing fillers, as shown in Figure 4.











Figure 4. Scheme of the recycling process [30–32] aimed at converting PS into ingredients for the production of sealants.

The phase of drying had to be preferably carried out by agitating the raw sludge under vacuum at a temperature of approx. 110 °C. These operating conditions determined the removal of a substantial amount of water and low-boiling solvents from the PS, thus

leaving the polymers uncured. In the second phase, PS underwent a treatment of de-ca- talysis with an alkali agent (which was selected from di-ethanolamine, 2-amino-2-methyl- 2-propanol, di-isopropanol amine, tri-isopropanol amine, potassium hydroxide (KOH), and sodium hydroxide (NaOH)) capable of guaranteeing a pH ranging from 8 to 13. The dose of the added alkali agent ranged from 0.1% to 10% of the PS's dry weight. In the de- catalysis phase, the catalyst contained in the PS was neutralized so that the curing com- ponent was not activated upon heating. In the presence of a solvent-based PS containing polyester, acrylic, and melamine resins, the above-described process produced a putty. Finally, in the third phase, putties were converted into powders by the addition of 5–75%
b.w. of processing fillers, namely black carbon (BC), clay, or a mixture thereof.
Furthermore, in this case, the success of recycling PS in the production process of sealants seems to be dependent on both the correct choice of the reagent to be used for the de-catalysis operation and the homogeneity of the waste in terms of composition. These requirements, together with the costs of reagents, had a strong impact on limiting its in- dustrialization.

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