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D

In water treatment programs, deposit control polymers (DCPs) are critical components that function as scale in-hibitors, crystal modifiers, and dis-persants. To remain effective, DCPs used in high-temperature applica-tions should sustain performance un-der a range of system operating con-ditions. This study evaluates the effect of thermal stress on DCP archi-tecture and performance.

Deposit control polymers (DCPs) as water treatment (WT) program compo-nents have multifunctional roles, including particulate dispersion, scale inhibition, hardness and metal ion stabilization, and sludge conditioning. DCPs used in WT programs for high-temperature applica-tions should be able to retain activity when exposed to system operating conditions.

In 1982, Masler presented thermal sta-bility results for several homopolymers (i.e., polyacrylic acids [PAAs], polymethacrylic acids [PMAAs], and polymaleic acids [PMAs]) commonly used in boiler applica-tions.1 Before and after thermal stress, homopolymers were (a) characterized by various analytical techniques and (b) evalu-ated as calcium carbonate (CaCO3) inhibi-tors. Under the conditions employed (5% active polymer solutions, pH 10.5, and 250 °C [594 psig] for 18 h), PAA, PMAA, and PMA all underwent some structural changes. In terms of molecular weight (MW) loss, PMAA lost slightly less than PAA, which lost considerably less than PMA. As CaCO3 inhibitors, PAA and PMAA had mini-mal performance changes, whereas PMA performance was reduced substantially.

Gurkayanak, et al.2 performed very short degradation tests on a 6,000-MW PAA at high temperatures and different pH lev-els. Their study showed that the decarbox-ylation rate depends upon solution pH, ionic strength, and temperature. Another study by Lepine and Gilbert3 on PAA ther-mal stress between 180 and 260 °C showed that degradation involves both decarboxyl-ation and polymer chain integrity.

DCP thermal stability testing results were recently presented to technical orga-nizations, including the Association of Water Technologies (AWT) and NACE International. These studies showed that DCP performance following thermal stress is affected both by temperature level and exposure time.4-6 This article presents ther-mal stress effects on DCP structure and performance. The DCPs evaluated include PAAs made by several processes, a PMAA, a PMA, acrylic acid/sulfonic acid copoly-mers, and acrylic acid/sulfonic acid-based terpolymers containing different third monomer groups. DCP compositions and MWs were characterized and performance was evaluated before and after thermal stress. The objective was to provide indus-trial water technologists guidance for selecting DCPs as components of high-temperature application WT programs.

Experimental ProcedureTable 1 identifies the polymers (e.g.,

Carbosperse† K-700 polymers, DCPs con-taining AMPS† monomer [SA], and other DCPs) that were tested. The stock polymer solutions were prepared in distilled water on an active solids basis.

Thermal Stress Effects on Deposit Control Polymers

Zahid amjad and RobeRt W. Zuhl, The Lubrizol Corp., Wickliffe, Ohio

†Trade name.

55NACE INTERNATIONAL: VOL. 54, NO. 9 MATERIALS PERFORMANCE SEPTEMBER 2015

The protocol to simulate DCP thermal stress has been previously discussed.5 The DCP thermal stress test conditions include 10% active polymer solutions, pH 10.5, nil oxygen, thermal stress (either 150 °C [84  psig], 200 °C [225  psig], or 250 °C [594 psig]) for 20 h. After thermal stress, the DCP samples were cooled to room temperature and solutions transferred to vials for characterization and performance testing.

DCP MWs were determined using gel permeation chromatography (GPC). DCP nuclear magnetic resonance (NMR) spectra before and after thermal stress were obtained on a Bruker† AV-500 NMR spec-trometer.

DCP CaCO3 threshold inhibition effi-cacy was evaluated using a previously described procedure.6 The conditions are summarized in Table 2.

DCP CaCO3 threshold inhibition effi-cacy was calculated using Equation (1).

= ×+ +

+ +% Inhibition[Ca ] –[Ca ][Ca ] –[Ca ]

1002

sample2

control2

initial2

control (1)

Equation (1) terms are defined as fol-lows: % Inhibition = % CaCO3 inhibition; [Ca2+]sample = Ca2+ concentration with inhibi-tor at test conclusion; [Ca2+]control = Ca2+ con-centration without inhibitor at test conclu-

sion; and [Ca2+]initial = Ca2+ concentration at test initiation.

ResultsPolymer Characterizations

All DCPs undergo changes caused by exposure to elevated temperature and pres-sure. These changes depend upon thermal stress magnitude and duration, as well as DCP composition. Observations made before and after thermal stress include (1) DCP color intensity increased with greater thermal stress, (2) PAA and PMAA sample solutions remained clear, and (3) the PMA and co/terpolymer samples became increasingly turbid, and precipitate forma-tion occurred.

Molecular WeightGPC MW distribution curves, before and

after thermal stress at 250 °C, were prepared and analyzed for the DCPs shown in Table 1. All DCPs evaluated exhibited MW losses that

increase with thermal stress. For example, the SA-based co/terpolymers MW losses increased markedly above 200 °C, suggesting some scisson and polymer backbone.

Table 3 summarizes DCP MW losses. Comparing homopolymer values before and after 250 °C thermal stress led to the observation that PAA and PMAA MW losses were all <7%, whereas PMA MW loss was ≈8%. In comparison, the MW losses due to thermal stress for both the AA/SA-based co/terpolymers were >10% at ≥200 °C with relative losses varying depending upon the initial MW and DCP composition.

DecarboxylationDCP scale inhibition and dispersant

performance depends on the DCP’s MW and amounts and types of functional groups. D C Ps c ontaining carboxyl (-COOH) groups exhibit excellent scale in-hibition performance. DCPs containing bulkier, hydrophobic, and strong acidic

TABLE 1. POLYMERS EVALUATEDProduct Composition MW Acronym

CK752 Solvent polymerized poly(acrylic acid) or “SPPAA” 2 k PAS1

CK7028 Water polymerized PAA or “WPPAA” 2.3 k PAW1

CK732 SPPAA 6 k PAS2

CK7058 WPPAA 7.3 k PAW2

CPAAP PAA with phosphinate groups <4 k PAAP

CPMA Poly(maleic acid) or “PMA” <1 k PMA

CK766 Sodium polymethacrylate or “PMAA” 5 k PMAA

CK775 Poly(acrylic acid: [2-acrylamido-2-methylpropane sulfonic acid]) or poly(AA/SA) with 74/26 monomer weight ratio

<15 k CP1

CK776 Poly(AA/SA) with 60/40 monomer weight ratio >10 k CP2

CK781 Poly(AA:SA: sulfonated styrene) or poly(AA/SA/SS) <10 k TP1

CK797 Poly(AA/SA/SS) <15 k TP2

CK798 Poly(AA/SA/SS) <15 k TP3

CTPD Poly(AA/SA/non-ionic) or “AA/SA/NI” 4.5 k TP4

TABLE 2. CaCO3 INHIBITION TEST CONDITIONSParameter mg/L Parameter Value

Calcium (Ca2+) 560 as CaCO3 Calcite saturation (LSI) 56x (1.89)

Bicarbonate (HCO3)– 630 as CaCO3 pH ≈8.3

Carbonate (CO3)2– 30 as CaCO3 Temperature 66 °C

Active polymer 0 to 5.0 Time 24 h

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groups provide better particulate disper-sion performance. DCP decarboxylation causes carboxyl content loss that de-creases efficacy as water treatment pro-gram components.

NMR CharacterizationDCP proton NMR spectra as a function

of thermal stress were prepared for PAW1,

PMAA, PMA, CP1, and TP1. However, due to space constraints, only the CP1 NMR spec-tra are presented and discussed.

The CP1 (AA/SA copolymer) proton NMR spectra (Figure 1) indicate stability up to 150 °C. However, the 200 and 250 °C NMR spectra show a side chain SA comonomer hydrolysis causing additional PAA forma-tion in the backbone. Despite thermal stress

changes, the original AA/SA copolymer end groups remain intact and the actual copoly-mer length does not appear to change.

CaCO3 InhibitionDCP CaCO3 threshold inhibition effi-

cacy is an important criterion for water treatment program components. Further-more, DCP carboxyl content is well known to affect CaCO3 inhibitor efficacy. Thermal stress may cause DCP carboxyl content changes that affect CaCO3 inhibition per-formance as shown in Figures 2 and 3.

Figure 2 presents homopolymer CaCO3 inhibition data vs. thermal stress that suggest:

• Thermal stress has a limited adverse performance effect for PAAs and PMAA.

• Before thermal stress, PMA provided the best (slightly better than PAS1) CaCO 3 inhibition performance. However, thermal stress significantly degraded PMA performance.

• The PAA that contained phosphinate end groups (PAAP) provided perfor-mance comparable to other PAAs.

• Both polymerization process and MW affect PAA CaCO3 inhibition performance. Table 4 compares the MW and CaCO3 inhibition perfor-mance for the five PAAs evaluated and the data indicate that the CaCO3 inhibition performance is inversely related to PAA MW (e.g., the lowest MW PAA performs best).

For the five inhibitors listed, perfor-mance for control of MW loss was in reverse order compared to performance for CaCO3 inhibition.

When comparing homopolymer CaCO3 inhibition performances, it is important to recognize differences between DCPs. PMA is made under different polymerization conditions, is structurally dissimilar (i.e., two carboxyl groups present on the adja-cent carbon atoms compared to only one carboxyl group present in the PAAs), and is much lower MW than the other homopoly-mers. However, the study data indicate that PMA is not as thermally stable as either the PAAs or PMAA. These changes provide one

TABLE 3. POLYMER MW LOSS CAUSED BY THERMAL STRESS

PolymerMW Loss @ 150 °C (%)

MW Loss @ 200 °C (%)

MW Loss @ 250 °C (%)

PAS1 1.0 2.6 4.3

PAW1 0.8 2.5 0.6

PAS2 0.0 2.3 1.2

PAW2 0.6 7.9 6.8

PAAP 0.1 4.6 3.9

PMAA 1.0 2.5 4.0

PMA 0.7 1.5 7.5

CP1 3.3 10.9 12.9

CP2 6.4 21.2 29.0

TP1 3.1 11.1 11.5

TP2 2.0 13.4 14.3

TP3 7.7 17.1 18.5

TP4 5.1 12.7 13.1

FIGURE 1 CP1 proton NMR spectra vs. thermal stress.

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explanation for Figure 2 data, indicating that thermally stressed PMA retains <50% of its pre-stress efficacy.

Figure 3 presents copolymer (CP1 and CP2) and terpolymer (TP1, TP2, TP3, and TP4) CaCO3 inhibition data as a function of thermal stress exposure. The data lead to the following observations and explanations:

• Before thermal stress, CaCO3 inhibi-tion values for the AA/SA-based co/terpolymers are less than that for the homopolymers except PMAA.

• Co/terpolymer CaCO3 inhibition per-formance increases as a function of thermal stress, presumably due to in-creases in carboxyl content.

• Both co/terpolymer carboxyl content and MW affect CaCO3 inhibition val-ues as summarized in Table 5.

ConclusionsConclusions drawn from this work are

as follows:1. DCPs exposed to thermal stress up to

250 °C undergo some degradation and/or compositional changes.

2. Homopolymer MW losses at 250 °C were all <8%, with PMA having the greatest loss.

3. Thermal stress MW losses for the SA-based co/terpolymers were >11% at ≥200 °C and >12% at 250 °C.

4. Thermal stress DCP proton NMR characterizations indicate the follow-ing changes:a. The PAAs and PMAA showed only

slight structural changes.b. The PMA showed significant struc-

tural changes.c. The SA-based copolymers showed

limited structural changes due to thermal stress ≤150 °C. However, thermal stress ≥200 °C caused the SA comonomer to hydrolyze to AA.

d. TP1, which has a higher SA content and contains SS co-monomer, is more thermally stable than CP1.

5. Thermal stress <250 °C has limited effects on the CaCO3 inhibitory prop-erties for PAAs and PMAA, whereas PMA performance decreased by ~50% vs. pre-stress conditions.

FIGURE 2 CaCO3 inhibition by homopolymers vs. thermal stress.

FIGURE 3 CaCO3 inhibition by co/terpolymers vs. thermal stress.

TABLE 5. POST THERMAL STRESS CO/TERPOLYMER CARBOXYL

CONTENT, MW, AND CaCO3 INHIBITION COMPARISONSParameter Copolymer Ranking Terpolymer Ranking

Carboxyl content CP1 > CP2 TP2 > TP1 ≈ TP3 ≈ TP4

MW CP2 > CP1 TP3 > TP2 > TP1 > TP4

CaCO3 inhibition CP1 > CP2 TP2 > TP1 > TP4 ≈ TP3

TABLE 4. POST THERMAL STRESS PAA MW AND CaCO3 INHIBITION

COMPARISONSParameter Polymer Ranking (highest to lowest)

MW PAW2 > PAS2 > PAAP > PAW1 ≈ PAS1

CaCO3 inhibition PAS1 > PAW1 > PAAP > PAS2 ≈ PAW2

Thermal Stress Effects on Deposit Control Polymers

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6. Co/terpolymer CaCO3 inhibition data as a function of thermal stress led to several observations and explana-tions:a. Without thermal stress, the perfor-

mance of AA/SA-based co/terpoly-mers is not as good as the homo-polymers tested (except for PMAA), which suggests that both DCP carboxyl content and MW affect performance.

b. Performance improves as a func-tion of thermal stress for all AA/SA-based co/terpolymers evaluated.

c. Both DCP carboxyl content and MW af fect AA/SA-based co/ terpolymer performance.

AcknowledgmentsThe authors thank our colleagues for

their contributions and The Lubrizol Corporation for permission and support to conduct this study.

References1 W.F. Masler, “Characterization and Thermal

Stability of Polymers for Boiler Treatment,” 43rd Annual Meeting, International Water Conference (Pittsburgh, PA: October, 1982).

2 A. Gurkayanak, F. Tubert, I. Yang, I. Matyas, I.L. Spencer, C.C. Gryte, “High Temperature Degradation of Poly(acrylic acid) in Aqueous Solution,” J. Polymer Science: Part A: Polymer Chemistry 34 (1996): pp. 349-355.

3 L. Lepine, R. Gilbert, “Thermal Degradation of Poly(acrylic acid) in Dilute Aqueous Solu-tion,” Polymer Degradation and Stability 75 (2002): pp. 337-345.

4 R.W. Zuhl, “New Boiler Water Treatment Polymer Developments,” 1990 Association of Water Technologies Spring Conference (Rockville, MD: AWT, 1990).

5 Z. Amjad, R.W. Zuhl, “An Evaluation of Silica Scale Control Additive for Industrial Water Systems,” CORROSION 2008 paper no. 08368 (Houston, TX: NACE International, 2008).

6 Z. Amjad, J.F. Zibrida, R.W. Zuhl, “The Im-pact of Thermal Stress on Deposit Control Agent Performance,” Association of Water Technologies 2013 Annual Convention (Rockville, MD: AWT, 2013).

This article is based on CORROSION 2014 paper no. 4457, presented in San Antonio, Texas.

ZAHID AMJAD is a technical consultant at The Lubrizol Corporation, 29400 Lakeland Blvd., Wickliffe, OH 44092, e-mail: zahid.amjad@lubrizol.com. He has an M.S. degree from Punjab University, Pakistan; a Ph.D. from Glasgow University, U.K.; and is a post-doctoral fellow at the State University of New York at Buffalo. A member of NACE International for more than 25 years, Amjad is also a member of the American Chemical Society, was inducted in the National Hall of Corporate Inventors, and is listed in American Men and Women of Sciences and Who’s Who of American Inventors. He received the Association of Water Technologies’ 2002 Ray Baum Memorial Water Technologist of the Year. He holds 30 U.S. patents, has published more than 150 technical papers and articles, and has edited eight books. He is a member of the MP Editorial Advisory Board.

BOB ZUHL is the global business manager, Water Treatment Chemicals, at The Lubrizol Corporation, e-mail: bob.zuhl@lubrizol.com. He has a B.S. degree in civil engineering and an M.S. degree in environmental engineering from Michigan State University, and an M.B.A. from Baldwin Wallace University. A member of NACE for more than 25 years, Zuhl is a reg is tered Profess iona l Eng ineer (Michigan), holds one patent, and has published more than 25 technical papers and articles.