High-build alkyd urethane coating materials with a partially solvolyzed waste polyurethane foam.
The amount of postconsumer plastic waste collected in Europe in 2011 reached about 25.1 million tons (2.4% of increment in relation to 2010) . Considering the average weight content of a flexible polyurethane foam (PUR) in a typical car and disassembled vehicles value in Poland, it can be calculated that only in that way about 35,000 tons of PUR waste were collected in this country in 2012 (30,600 tons in 2011) [2, 3], Research studies and tests have resulted in a number of recycling and recovery methods, which are economically and environmentally viable for PUR waste [4, 5]. There are four major categories of recycling, namely, mechanical, chemical and thermomechanical recycling, energy recovery, and product recycling . Two former groups of PUR recycling are based on a physical treatment (material recycling) and a chemical treatment. The chemical treatment produces feedstock chemicals for chemical processing. One of the mechanical recycling options is regrinding PUR foams into powders allowing them to be reused as fillers in a new PUR foam. Cryogenic grinding of PUR foams is well known ; however, it was found out that two-roll grinding of flexible PUR foams gives the best results , Cannon Company in cooperation with Mobius Technology (a developer of a PUR foam powder with particle size ca. 50 pm) commercialized the production of new flexible molded foams with PUR powder for automotive applications ,
When it comes to chemical recycling of PUR waste, the most important technique seems to be glycolysis frequently performed by batch methods. Such a process is usually conducted at temperatures above 200[degrees]C by means of diethylene glycol with some coreagents like diethanolamine  or catalysts . After several hours, one- or two-phase polyol product is obtained [4, 5]. The aim of this process involves recovery of polyols for the production of a new PUR material.
Total glycolysis or aminoglycolysis of PUR waste can also be conducted in a continuous way in an extruder [10, 11]. These chemical recycling processes can be substantially shortened from several to 20 min while the temperature is usually kept between 175 and 250[degrees]C.
On the basis of the data found in literature, we have developed a continuous method of partial glycolysis/aminoglycolysis of a soft waste PUR foam in an extruder. Extrusion products containing solid particles with OH groups were tested as reactive modifiers of alkyd urethane 2K coating compositions applied on a steel substrate. The main object of the presented research is to investigate the influence of a partially solvolyzed waste PUR foam addition on application, mechanical and barrier attributes of prepared paints, and cured high-build coats designed for car underbody protection against stone chips.
MATERIALS AND METHODS
The following commercial components were used to prepare solvent-borne alkyd urethane coating compositions: WorleeKyd C628, 70% solution in xylene of short oil nondrying hydroxyalkyd, oil content 40%, and OH group content of about 2.2% (Worlee-Chemie GmbH, Germany); Tolonate HDB-LV, hexamethylene diisocyanate derivative, viscosity about of 2000 mPa s at 25[degrees]C, and NCO content 23.5 wt% (Perstorp AB, Sweden); wetting/dispersing additive BYK-P 104S and silicone defoamer BYK-067A (BYK-Chemie GmbH, Germany); and xylene as a solvent (POCh S.A., Poland). Waste soft PUR foam based on tolylene diisocyanate and polyether polyol with molecular weight of about 3600 g [mol.sup.-1] (Megaflex-Schaumstoff GmbH, Germany) was delivered by Bossmebel, Poland. Technical-grade trimethylolpropane (TMP) and tris(hydroxymethyl)aminomethane (TRIS) applied as solvolytic agents of a PUR foam were purchased from Sigma-Aldrich (Germany).
Pulverization and Partial Solvolysis of a Waste PUR Foam
The foam was preliminarily disintegrated by using a threshing machine and then grounded in a twin-screw extruder (D = 16 mm, L/D = 40; Prism Thermo Electron, UK) at room temperature. Next, partial solvolysis of the PUR powder (particle size < 1 mm, PUR/E) with either TMP or TRIS was performed in the extruder at 180-200[degrees]C (PUR/TMP) and 160-180[degrees]C (PUR/TRIS) with the screw speed of 50 rpm , The composition and characterization of partially solvolyzed waste PUR foam are presented in Table 1.
Preparation of Coating Compositions
WorleeKyd C628, BYK-P 104S (2wt parts/100 wt parts of PUR foam extrusion product), BYK 067A (0.2 wt part/100 wt parts of paint solids), and xylene were mixed for 10 min by using a laboratory dissolver (1000 rpm; VMA Getzmann GmbH, Germany). Then, the extruded polyurethane foam (PUR/E, PUR/TMP, or PUR/TRIS) was incorporated into this mixture, and the system was mixed at 2000 rpm for 1 h. Such a prepared composition was diluted with xylene to 65% of total solids, filtered (500 pm), and mixed with a diisocyanate hardener using a laboratory mixer. A dose of curing agent was calculated on the basis of hydroxyl numbers of a binder and extrusion product. The composition of alkyd urethane paints is presented in Table 2.
Paints were applied with a brush and leveled with a gap film applicator (Dozafil, Poland) onto steel panels (76 mm X 152 mm; Q-Panels, Q-Lab, England) or onto a PET foil and cured at room temperature for 21 days. The thickness of wet coats was as follows: 0.7 mm for an adhesion test, 1 mm for electrochemical measurements, an indentation test, and a gravimetric test of distilled water absorption (WA), as well as 1.55 mm for the rest of experiments.
Characterization of Powdered Polyurethane Foam and Solvolysis Products
Hydroxyl number of extrusion products was measured according to the Polish standard PN-C-89052-03:1993 (colorimetric titration of a sample dispersed in a mixture of anhydrous acetone, phthalic anhydride, and phenolphthalein; an aqueous NaOH solution was used as a titrant). Acid value of the sample was determined according to PN-C-89052-02:1985 (colorimetric analysis of the sample dispersed in an aqueous solution of bromothymol blue; an aqueous NaOH solution was used as a titrant) using an automatic titrator (T50; Mettler Toledo).
Partially solvolyzed PUR particles were separated from an extrusion product by sedimentation. Extrusion product was diluted with methanol and then centrifuged for 30 min (3000 rpm). The deposit was washed several times using methanol and dried at 40[degrees]C.
The Fourier transform infrared spectroscopic (FTIR) analysis of extrusion products and partially solvolyzed PUR particles was made by using the Nexus FTIR spectroscope with attenuated total reflectance accessories (Thermo Nicolet).
Particle size analysis of a partially solvolyzed PUR foam was made by using the Laser Scanning Microscope VK-9700 (Keyence). The samples were dispersed in a commercial polyol mixture before the test.
Characterization of Coating Compositions and Cured Paints
Viscosity of coating compositions without the hardener and viscosity of ready-to-use paints (after 15 min since mixing with the hardener) were determined at 10 rpm of a spindle by using the Brookfield RV viscometer (Brookfield Engineering Laboratories). Pull-off adhesion of coats to a steel substrate (PN-EN ISO 4624:2004) as well as tensile experiments for alkyd urethane films (prepared on a PET foil) were made by using Instron 4026 machine (Instron). Hardness of cured coats (Barcol hardness tester, 23[degrees]C, PN-EN 59), their abrasion resistance (ASTM D-968, a falling sand abrasion method), and a cupping test (PN-EN ISO 1520) were also performed. A gravimetrical test of distilled WA of dry coats was carried out by using coated steel samples with edges and a back side protected with a paraffin wax. Weighed samples were carefully placed into distilled water and stored at room temperature. The weight of the samples was controlled every day (after drying by means of a blotting paper) for 15 days. Gravimetrically determined WA was calculated according to the following equation:
WA= 100([m.sub.t] - [m.sub.p])/([m.sub.c] - m)(wt%) (1)
where m is the mass of a steel substrate, mc is the mass of a coated steel substrate, mp is the mass of the coated steel substrate protected with a wax, and mt is the mass of the coated/ protected steel substrate after t-time immersion in distilled water.
The dynamic indentation test of a cured coat on a steel substrate (simulation of a stone chip stroke) was made by using a pneumatic air gun and a steel ball (9 mm of diameter, mass 3 g, and velocity 100 [+ or -] 1 m [s.sup.-1]).
The thickness of the cured films was measured with electronic film gauge Byko-test 8500 (BYK-Gardner) according to PN-EN ISO 2808. The glass transition temperature of cured paints as the temperature of tan delta peak was evaluated by using a dynamic mechanical analyzer Q800 (tension test in the range of -80[degrees]C to 40[degrees]C; heating rate: 3[degrees]C [min.sup.-1]; frequency: 1 Hz; and amplitude: 15 [micro]m; TA Instruments). Their thermostability, that is, the temperature of 5 and 20% mass loss, was determined with thermogravimetric analyzer Q500 (25-600[degrees]C, 10[degrees]C [min.sup.-1], and air atmosphere; TA Instruments).
Digital images of a cured polyurethane coat fracture were made by using Laser Scanning Microscope VK-9700.
The barrier properties of cured paints on a steel substrate were investigated by means of an electrochemical impedance spectroscopic (EIS) technique. EIS tests were carried out on coated panels (one-layer coatings with 587-710 [micro]m thickness) after 0, 50, and 250 h of their immersion in a periodically air-saturated aqueous NaCl solution (3.5 wt%). These measurements were conducted by using three coated samples for each tested composition. A three-electrode glass cell (with 14.5 [cm.sup.2] surface sample area) equipped with a graphite counter electrode and a saturated calomel reference electrode was used inside the Faraday cage. The impedance data (at frequency 0.001-10,000 Hz, 100 mV amplitude of sinusoidal voltage vs. open circuit potential) was collected by EIS300 software with FAS2 femtostat (Gamry) and analyzed by using the electric circuit models presented in Fig. 1 ([R.sub.u], uncompensated solution resistance; [R.sub.c], coat resistance; [C.sub.c], coat capacitance; [C.sub.f], double layer capacitance; Rf, polarization resistance) . Coat resistance and coat capacitance values were computed in respect to coat thickness and presented as a relative [R.sub.c] (i.e., [R.sub.cr], [OMEGA] [micro][m.sup.-1]) and relative Cc ([C.sub.cr], F [micro][m.sup.-1]).
RESULTS AND DISCUSSION
Partial Solvolysis of PUR Foam and Viscosity of Coating Compositions
It is well known that during a PUR glycolysis process, a solvolytic agent reacts with polyurethane bonds and mainly polyol component (used for the foam preparation) is recovered (Fig. 2a). A similar reaction between polyurethane linkages and primary or secondary amine groups may occur during aminoglycolysis or aminolysis of PUR (carbamate linkages are produced) [5, 14]. In the case of solvolysis of the investigated waste PUR foam, a polyol component (i.e., polyether polyol and its derivatives) should be produced in the amount depending on TMP or TRIS content in an extruded system. It means that hydroxyl number values (HVs) for a PUR/solvolytic agent mixture and for an extrusion product should be alike. The comparison of theoretical and measured HV for the investigated samples shows that these values are rather similar (relatively high HV noted for PUR/E foam was probably caused by a presence of hydroxyl groups in a structure of the foam and was taken into account in the calculation of theoretical HV for PUR/TMP and PUR/TRIS products; Table 1). It can be observed that HV measured for PUR/TMP-10 was significantly higher (126 mg KOH [g.sup.-1]) than for PUR/TMP-5 product (88 mg KOH [g.sup.-1]), and moreover, the former extrusion product contained a lower solid content (Table 1). Polyol component and its derivatives (produced during PUR solvolysis) acted as a thinner of a coating composition, and thus, samples containing PUR/TMP-10 exhibited lower viscosity than systems with PUR/E or PUR/TMP-5 (Fig. 3). On the other hand, PUR/TRIS-5 contained only 45 wt% of solids; however, coating compositions filled with this extrusion product reached a higher viscosity than a sample containing PUR/TMP-10 (53 wt% of solids). Probably, it was resulted by a higher functionality and reactivity of TRIS (three OH and one NH2 groups in the molecule) in comparison with TMP (three OH groups). Thus, PUR/TRIS-5 might contain more branched and more viscous solvolysis products (polyether polyol and TRIS derivatives) than PUR/TMP systems. Moreover, PUR/TR1S extrusion product might contain urethane as well as rigid urea linkages. It should be noted that viscosity of WK/PS5 systems might also be affected by the size of partially solvolyzed PUR particles. The analysis of extruded materials showed that the particle size of PUR/TRIS-5 was much below about 160 [micro]m, whereas the particle size of PUR/TMP-10 was higher (<200 [micro]m; Fig. 4). It is known that small particles of an inorganic filler often exhibit a high oil absorption value (higher than larger particles), and it resulted in an increment of paint viscosity.
Figure 3 also presents viscosity values for ready-to-use paints (i.e., paints after 15 min since mixing with the hard ener) filled with waste PUR extrusion products. As it can be seen, paints containing 40 wt% of PUR/TRIS-5 exhibited extraordinarily higher viscosity (528 Pa s) than samples with similar content of other partially solvolyzed PUR foams (7.27.6 Pa s). Probably, high viscosity of WK/PS5-40 paint was caused by the presence of amine group in a molecule of a solvolytic agent and/or in PUR foam solvolysis products. TRIS and its derivatives might immediately react with an isocyanate hardener or catalyze a reaction of isocyanate and hydroxyl groups of a PUR foam extrusion product and a hydroxyalkyd binder, and thus, viscosity of the paint rose quickly after the addition of a hardener. FTIR spectra analysis revealed that PUR/TRIS-5 sample contained NH2 groups (peak at 3360 [cm.sup.-1]; Fig. 5c). These groups were not chemically bonded with partially solvolyzed PUR foam particles because the band was not observed on a spectrum of solid residue after solvolyzate extraction with methanol (Fig. 5e). Nevertheless, the spectra of PUR/TMP-5 and PUR/TRIS-5 (extrusion products and their solid parts; Fig. 5b-e) contained a relatively small peak at 3450 [cm.sup.-1], which represents secondary amine groups. Probably, these groups were generated during a urethane linkage transformation with carbon dioxide emanation (urea linkage cannot be directly transformed to a secondary amine group; Fig. 2b and c). The absorbance at 1730 [cm.sup.-1] (registered for PUR/TRIS-5; Fig. 5c) is characteristic for a free TRIS structure and was not observed for solids isolated from extruded PUR/TRIS-5 system (Fig. 5e). Instead of that band, a new one was recorded at 1725 [cm.sup.-1] corresponding to a carbonyl group in a urethane linkage .
From the comparison of IR bands with the peak at 3300 [cm.sup.-1], it is evident that partially solvolyzed PUR particles contained much more OH groups on the surface than the reference PUR/E sample (Fig. 5). It confirmed that surface functionalization of PUR particles effectively occurred during the extrusion with TMP or TRIS polyols.
Mechanical Properties of Cured Coats
Pull-of adhesion and hardness test results for alkyd urethane coats containing a partially solvolyzed PUR foam are showed in Fig. 6. The samples with PUR/TMP or PUR/TRIS extrusion products exhibited a markedly lower value of the former parameter in respect to WK/P. The adhesion of PUR/E-based coat to a steel substrate amounted to 5.3 MPa, whereas samples with a partially solvolyzed PUR foam reached only 4.1-4.6 MPa. In the authors' opinion, lower adhesion of coats containing the latter additive was caused by the presence of polyether polyol and its derivatives in an extrusion product. It is generally known that polyether polyols (mainly with a high molecular weight) effectively reduce adhesion of polyurethane adhesives and coats to a steel substrate .
As it can be seen, the coats filled with a lower amount of extrusion products (i.e., 20 wt%) reached a higher hardness (2836 HBa) than the WK/P sample (26 HBa; Fig. 6). On the other hand, the incorporation of 40 wt% of a partially solvolyzed PUR foam into the paint markedly decreased that parameter value (due to an elastic character of PUR particles). Interestingly, the highest and the lowest hardness values (among the samples with PUR/TMP and PUR/TRIS) were registered for samples with PUR/TMP-5, that is, WK/PP5-20 (36 HBa) and WK/PP5-40 (19 HBa), respectively. Probably, a relatively high HV measured for PUR/TMP-5 (in comparison with PUR/E and PUR/TRIS-5) resulted in an increment of crosslinking density of a coat matrix (high hardness of WK/PP5-20 coat). On the other hand, PUR/TMP-5 contained a larger amount of an unsolvolyzed PUR foam than PUR/TMP-10 and PUR/TRIS-5, and thus, WK/PP5-40 sample (consisting highly crosslinked polymeric matrix as well as a large amount of partially solvolyzed soft PUR particles) reached lower hardness than WK/PP10-40 and WK/PS5-40.
Upgraded abrasion resistance of cured coats (in respect to WK/P, 2850 g [micro][m.sup.-1]; Fig. 7) was observed for the samples with PUR/TRIS-5 (3188 g [micro][m.sup.-1] for WK/PS5-20 and 3207g [micro][m.sup.-1] for WK/PS5-40) or with a higher amount of PUR/TMP-5 (3185g [micro][m.sup.-1] for WK/PP5-40). In the case of materials containing 20wt% of extrusion product, the attrition partly correlates with the hardness of coats; the lowest abrasion resistance was registered for samples with the highest hardness (2219 g [micro][m.sup.-1] for WK/PP5-20). Moreover, abrasion resistance of WK/PP- and WK/PS-type samples corresponds to a strain at break values recorded for alkyd urethane free films (Fig. 7). Generally, the higher the value of the latter parameter, the higher is the abrasion resistance. It is generally known that flexible materials (e.g., elastomers) often exhibit higher abrasion resistance than the brittle ones.
It seems that cupping test results should directly depend on deformability of the cured coat. Nevertheless, the measured indentation depth values of coats (Fig. 8) containing an extruded PUR foam do not correspond with strain at break of free films (Fig. 7). As can be observed, only WK/PP5-20 as well as WK/ PP10-20 coats reached a subtly higher indentation depth than WK/P and other systems. This parameter was reduced after the addition of PUR/TRIS-5 (20 and 40 wt%) or PUR/TMP products (40 wt%). Probably, cupping test results of the cured coat containing a partially solvolyzed PUR foam depend on the paint viscosity. It should be noted that the aforementioned systems (i.e., WK/PP5-20 as well as WK/PP10-20) were characterized by lower viscosity before curing in comparison with WK/P and other paints. High viscosity of a paint may limit its leveling and/or deaeration during a drying process. In this case, the surface defects and occluded microsized air bubbles in a relatively thin cured coat (0.65 mm) resulted in a decrement of elongation at break (thus lower indentation depth was noted). On the other hand, this phenomenon was not crucial for tensile strength of a thicker film (1 mm; Fig. 8). As it can be observed, the tensile strength values for WK/P and most coats containing a partially solvolyzed PUR foam were in a narrow range (9.0-11.6 MPa). Only WK/PS5-40 film exhibited a markedly lower value of that parameter (7.6 MPa). Perhaps it was affected by extraordinary high viscosity of WK/PS5-40 paint (Fig. 3) and/or relatively low solids content and a low HV value of PUR/TRIS-5 (45 wt% and 73 mg KOH [g.sup.-1] Table 1) in relation to the other extrusion products. Indeed, the solvolysis products, that is, a polyol component recovered during the PUR solvolysis using TRIS, should consist of high-molecular-weight derivatives of the initial polyether polyol. It means that crosslinking density of the matrix in WK/PS5-40 sample (containing the mentioned high-molecular-weight component) should be lower in comparison with the samples with other extrusion products. This resulted in relatively low tensile strength of WK/PS-40 (and high strain at break; Fig. 7) in relation to the rest of the samples containing a partially solvolyzed PUR foam. Tensile strength of films containing PUR/TMP systems depends directly on HV of the extrusion product as well (Fig. 8 and Table 1). In that case, the higher the HV, the higher is the tensile strength of a cured sample. These extrusion products contained a similar amount of PUR solids (64 and 53 wt%, respectively); however, PUR/TMP10 reached markedly higher HV than PUR/TMP-5. It resulted in higher crosslinking density of samples containing the former partially solvolyzed PUR foam.
Dynamic indentation test results are presented in Fig. 9. The samples containing PUR/E or a lower amount of a partially solvolyzed PUR foam were damaged during the test (large cracks and delamination between steel and coat were detected). In the case of materials with higher content of extrusion product, WK/ PP5-40 and WK/PP10-40 samples exhibited only a slight adhesion loss, whereas WK/PS5-40 was not damaged during the test. Generally, the dynamic indentation test results do not correlate with pull-off adhesion, hardness, cupping test, and tensile test results for the investigated systems. Probably, only partially solvolyzed PUR content and crosslinking density of the alkyd urethane matrix (PUR/TRIS-5 was characterized by lower HV and lower PUR particle content than PUR/TMP systems) are the crucial parameters for high-velocity impact denting resistance.
Fracture micrographs of WK/P and samples with 40 wt% of PUR/TMP or PUR/TRIS products showed that the structure of coats with a partially solvolyzed PUR foam was significantly more uniform (Fig. 10). Large, sharp-shaped PUR foam particles were visible only in a WK/P sample. Nevertheless, lower homogeneity of this material does not affect its mechanical properties.
Thermomechanical and Thermal Properties of Cured Coats
A cured sample containing 20 wt% of PUR/E or 40 wt% of an extrusion product exhibited two glass transition temperatures ([T.sub.g]; Table 2). Probably, Peak 1 corresponds to PUR foam particles, and Peak 2 represents the [T.sub.g] of the coat matrix. Generally, [T.sub.g] values for the samples containing a higher dose of PUR/ TMP or PUR/TRIS were slightly higher (Peak 1: from -45.3[degrees]C to -43.3[degrees]C) and markedly lower (Peak 2: 21.7-26.6[degrees]C) in comparison with WK/P (-48.2[degrees]C and 30.5[degrees]C, respectively) and other samples. Taking into consideration the [T.sub.g] value (Peak 2) for samples with 40 wt% of a partially solvolyzed PUR foam, it should be noted that the mentioned parameter depends directly on HV of an incorporated additive; the highest [T.sub.g] (i.e., 26.6[degrees]C) was observed for the specimen with PUR/TMP-10 (126 mg KOH [g.sup,-1]; Table 1). Moreover, tan delta values were lower for samples with the mentioned component (Fig. 11). It means that the alkyd urethane matrix of these samples (i.e., WK/PP10-20 and WK/PP10-40) exhibited generally higher rigidity (the storage modulus was upgraded) in comparison with the samples with either PUR/TMP-5 or PUR/TRIS-5. Liquid WK/PP10-40 paints contained higher amount of an isocyanate hardener, thus higher crosslinking density of a hydroxyalkyd/polyol component mixture around partially solvolyzed foam particles should be achieved in relation to other samples with similar amount of the extrusion product. Comparison of the samples containing 20 and 40 wt% of partially solvolyzed PUR foam shows that [T.sub.g] (Peak 2) is generally lower for the systems with higher additive content. It was resulted by the lower ratio of hydroxyalkydrecovered polyether polyol component (i.e., higher content of polyether polyol and its derivative) in the samples containing 40 wt% of extrusion products. In that case, the crosslinking density of the alkyd urethane matrix increased (higher rigidity); moreover, on the other hand, the content of elastic component (i.e., polyether polyol) also increased (lower [T.sub.g]). This phenomenon was recorded for samples with PUR/TRIS-5. The [T.sub.g] for these coats was reduced from 29[degrees]C (WK/PS5-20) to 21.7[degrees]C (WK/ PS5-40), and the tan delta was higher for WK/PS5-20 (Fig. 11).
The samples containing a lower dose of a partially solvolyzed PUR foam exhibited only one [T.sub.g] value (i.e., one tan delta peak; Peak 2). Probably, in these materials, the elastic and plastic features, registered during tension tests using the DMT A apparatus, depend mainly on a rigid matrix of the cured composition (the mechanical response of uniformly dispersed small particles was not recorded). The WK/P sample exhibited [T.sub.g] (Peak 1) because of the presence of a higher amount of large PUR particles in that material.
Interestingly, the dynamic indentation test results (Fig. 9) correspond to the presented [T.sub.g] values (Peak 2). As it can be seen, the lower the [T.sub.g] value of the sample, the higher is the dynamic indentation resistance. The undamaged coat (WK/PS540) as well as WK/PP5-40 (the lowest delamination effect) exhibited lower [T.sub.g] and higher elasticity (Fig. 11) in comparison with WK/PP 10-40.
Thermal stability (i.e., temperature at 5 and 20% mass loss) of cured coats with extrusion products is presented in Table 2. As it can be observed, these parameter values are lower for the samples with a higher dose of the additives (i.e., 274-279[degrees]C at 5% and 316-320[degrees]C at 20% mass loss) in comparison with samples containing 20 wt% of PUR/E (283/327[degrees]C) or chemically treated PUR foam (283-285[degrees]C/323-325[degrees]C). Taking into consideration the samples with 20 or 40 wt% of a partially solvolyzed PUR foam, it should be mentioned that the highest temperature (5% mass loss) was registered for coats containing PUR/TMP10; this additive was characterized by the highest hydroxyl value (126 mg KOH [g.sup.-1]; Table 1). In the case of 20% mass loss, such tendency was noted only for coats containing a higher dose of a partially solvolyzed foam; WK/PP10-40 lost 20 wt% at 320[degrees]C, whereas other samples lost 20 wt% at 316/318[degrees]C (WK/PS5-40 and WK/PP5-40, respectively).
Barrier Properties of Cured Coats
Distilled WA values of a cured coat are presented in Fig. 12. The analyzed parameter depends directly on the type of a solvolytic agent as well as on the content of the extrusion product in a coating composition. Taking into consideration the kind of tested polyols, systems filled with PUR/TR1S-5 exhibited markedly higher water uptake than coats with PUR/TMP products. Although TRIS and TMP are fully soluble in water, the former compound (containing three hydroxyl groups and one amine group) creates probably more hydrophilic PUR degradation products, which increase (even after crosslinking) the total hydrophilicity of a final coat. In the case of samples with PUR/ TMP, it can be observed that WK/PP10-20 (or WK/PP10-40) reached slightly higher WA value in comparison with WK/PP520 (WK/PP5-40). However, clearly, higher values of the analyzed parameter were registered for coats containing 40 wt% of PUR/TMP systems. Nevertheless, it should be mentioned that WA for WK/PP5-20 and WK/PP10-20 was higher than for WK/ P. Increasing water uptake was observed for all samples during the whole immersion test (Table 2); however, after a 15-day immersion in distilled water, none of the coats (with PUR/E, PUR/TMP, and PUR/TRIS) exhibited blistering or other surface changes.
Barrier features of the cured coats were tested using EIS technique as well. Relative coat resistance test results are presented in Fig. 13. Considering the [R.sub.cr] values after 250 h of immersion, it can be observed that the lowest conductivity (affected by a saline absorption in the analyzed material) was revealed for WK/PP10-40 and WK/PS5-40. On the other hand, these materials exhibited the lowest relative coating capacitance (Fig. 14) and the highest (distilled) WA (Fig. 12) in relation to other coating systems (WK/P reached the opposite values). This phenomenon was probably caused by the structure, extrusion product content, and hydrophilic character of the tested coats. WK/P system contained large particles of an untreated PUR foam (20 wt%), and thus, it was generally hydrophobic and slightly porous (Fig. 10), whereas WK/PP1040 and WK/PS5-40 included a higher amount (40 wt%) of a semihydrophilic partially solvolyzed PUR foam (and their structures were quite uniform). In the case of WK/P, a relatively large amount of pure water was collected in the pores (exchange of air into water occurred thus [C.sub.cr] increased; Fig. 14), and simultaneously, the saline was transmitted into the steel substrate (low [R.sub.cr] value was observed after 250 h of immersion; Fig. 13). WK/PP10-40 and WK/PS5-40 coats swelled during immersion (absorption of a large amount of pure water by hydrophilic coating components), and a saline transmission into the substrate was limited (relatively low [C.sub.cr] and high [R.sub.cr] values were registered after immersion; Figs. 13 and 14). Two time constants observed in Bode plots (not presented) for all tested coats (besides WK/PP10-40 and WK/ PS5-40) after their immersion in the saline confirmed the presented mechanism. In these samples, the coat/steel interface delamination occurred theoretically.
The summary of an influence of a partially solvolyzed PUR foam on several properties of coating materials (in relation to the reference sample based on 20 wt% of PUR/E) is presented in Table 3. As can be seen, chemically treated PUR foams significantly affect (in different ways) application, mechanical, and protective features of liquid paints and cured coats. The analyzed properties were most positively upgraded in the case of PUR/TMP-10 incorporation. A polyurethane foam partially solvolyzed by using 10 wt parts of TMP (per 100 wt parts of PUR) and used in 20 wt parts (per 80 wt parts of polymeric binder, i.e., hydroxyalkyd) decreased paint viscosity up to 33%, whereas the cured coats based on that material exhibited higher hardness (+8%), slightly better cupping test results (higher indentation depth, +1%), higher tensile strength (+11%), and thermal stability (+ 2[degrees]C) in comparison with WK/P sample. On the other hand, relatively low abrasion resistance (-17%) and pull-off adhesion to a steel substrate (-23%) belong to the main drawbacks of this coating system (WK/PP10-20). Nevertheless, taking into account the main application area of the analyzed alkyd urethane coats, for example, car underbody protection, values of the most important features (i.e., dynamic indentation resistance and abrasion resistance) are accepted only for coating systems containing 40 wt% of PUR/TMP-5 or PUR/TRIS-5. Considering the viscosity of the mentioned ready-to-use compositions, the former system seems to be the best alternative for forming high-build and waste PUR-based protective coats on a steel substrate.
Based on the test results of alkyd urethane coating compositions and coats modified with a partially solvolyzed PUR foam, the following conclusions can be drawn:
* PUR/TMP and PUR/TRIS extrusion products significantly affected the viscosity of a solvent-borne coating system. The lower values of this parameter were observed for samples containing 20 wt% of a PUR foam chemically treated with 5 or 10 wt parts of trimethylolpropane (per 100 wt parts of PUR).
* The presence of extrusion products influenced on pull-of adhesion, hardness, abrasion resistance, cupping test results, tensile strength, thermal stability, and glass transition temperature of cured coats in different ways. Moreover, the barrier properties of the coats (i.e., distilled WA and electrical resistivity after their immersion in an aqueous NaCl solution) depend on the type and content of the extrusion product.
* Generally, samples filled with 20 wt% of PUR/TMP-10 exhibited better mechanical properties (higher hardness, indentation depth, and tensile strength) in comparison with systems based on a chemically untreated PUR foam (PUR/E) and other extrusion products. Nevertheless, a good impact denting resistance of partially solvolyzed PUR-based coats (cured on a steel substrate) was achieved only for the samples containing 40 wt% of PUR/TMP or PUR/TRIS microfdlers. Probably, the crucial parameter for the mentioned feature is glass transition temperature of an alkyd urethane matrix.
The authors thank Ms. Agata Pukajlo for the preparation and characterization of waste polyurethane foam solvolysis products and Dr. Michal Barcikowski, PhD, for the examination of dynamic indentation resistance of cured coats.
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Krzysztof Kowalczyk, Tadeusz Spychaj, Grzegorz Krala
Polymer Institute, West Pomeranian University of Technology in Szczecin, Puiaskiego 10, 70-322 Szczecin, Poland
Correspondence to: K. Kowalczyk; e-mail: firstname.lastname@example.org
TABLE 1. Composition, hydroxyl number values, and solid content of a partially solvolyzed waste polyurethane foam. Extrusion Solvolytic PUR:solvolytic Hydroxyl Solid product agent agent ratio number (a) content acronym (wt parts) (mg KOH (a,b) [g.sup.-l]) (wt%) PUR/E -- -- 22 (c) 88 PUR/TMP-5 TMP 100:5 81 (d)/88 (c) 64 PUR/TMP-10 TMP 100:10 134 (d)/126 (c) 53 PUR/TRIS-5 TRIS 100:5 87 (d)/73 (c) 45 (a) Hydroxyl number and solid content of an extruded polyurethane foam. (b) Ref. . (c) Measured hydroxyl number value. (d) Theoretical hydroxyl number value. TABLE 2. Composition, thermal properties, and water absorption of alkyd urethane coats with a partially solvolyzed waste polyurethane foam. Paint Extrusion product Glass transition acronym temperature ([degrees]C) Acronym Dose Peak Peak (wt% (b)) 1 (c) 2 (c) WK/P PUR/E 20 -48.2 30.5 WK/PP5-20 PUR/TMP-5 20 -- 31.0 WK/PP10-20 PUR/TMP-10 20 -- 30.5 WK/PS5-20 PUR/TRIS-5 20 -- 29.0 WK/PP5-40 PUR/TMP-5 40 -43.3 23.8 WK/PP 10-40 PUR/TMP-10 40 -45.3 26.6 WK/PS5-40 PUR/TRIS-5 40 -45.1 21.7 Paint Weight loss Water acronym temperature ([degrees]C) absorption (a) (wt%) [T.sub.5.sup.d] [T.sub.20.sup.e] After After 10 15 days days WK/P 283 327 3.2 3.3 WK/PP5-20 283 325 3.5 3.8 WK/PP10-20 285 324 3.5 4.0 WK/PS5-20 283 323 4.5 5.2 WK/PP5-40 274 318 4.6 5.1 WK/PP 10-40 279 320 4.8 5.3 WK/PS5-40 274 316 7.1 8.3 (a) After immersion in distilled water for 10 and 15 days. (b) wt% of paint solids. (c) Peaks of tan delta curve. (d) Temperature at 5% mass loss. (e) Temperature at 20% mass loss. TABLE 3. Influence of partially solvolyzed PUR foam addition on properties of alkyd urethane coating materials. Coating material property Improvement index (a) (%) WK/PP5-20 WK/PP10-20 WK/PS5-20 Paint viscosity 27 33 -21 Hardness 38 8 8 Abrasion resistance -22 -17 12 Pull-off adhesion -16 -23 -13 Indentation (b) depth 6 1 -3 Dynamic indentation resistance 0 0 0 Tensile strength -14 11 -14 Water absorption resistance (d) -0.5 -0.7 -1.9 Thermal stability (e) 0 2 0 Coating material property Improvement index (a) (%) WK/PP5-40 WK/PP 10-40 WK/PS5-40 Paint viscosity -253 -240 -333,000 Hardness -27 -15 -8 Abrasion resistance 12 -4 13 Pull-off adhesion -19 -19 -15 Indentation (b) depth -3 -2 -2 Dynamic indentation resistance +' + + Tensile strength -8 -2 -27 Water absorption resistance (d) -1.8 -2.0 -5.0 Thermal stability (e) -9 -4 -9 (a) In relation to WK/P paint. (b) Static test results. (c) Relatively improved resistance. (d) After immersion in distilled water for 15 days; deterioration index in (wt%). (e) Improvement index in ([degrees]C).
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|Author:||Kowalczyk, Krzysztof; Spychaj, Tadeusz; Krala, Grzegorz|
|Publication:||Polymer Engineering and Science|
|Date:||Sep 1, 2015|
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