deposit control polymer selection criteria for high ......tp2 479 519 8 tp3 443 496 12 nmr...

Deposit Control Polymer Selection Criteria for High-Temperature Applications: Part 1—Polymer CharacterizationsBy Zahid Amjad, Ph.D., and Robert W. Zuhl, P.E., The Lubrizol Corporation and John F. Zibrida, ZIBEX, Inc.

40 the Analyst Volume 22 Number 3

AbstractDeposit control polymers (DCPs) play important roles in effective water treatment (WT) programs. DCPs func-tion as scale inhibitors, dispersants, and crystal modi-fiers, thereby conditioning precipitated solid surfaces so they are more readily kept in suspension and are less adherent to equipment surfaces. DCPs used as compo-nents of high-temperature WT programs should sustain performance under stressed operating conditions. This study correlates thermally stressed DCP performance with structural changes, as characterized by various analytical techniques.

IntroductionWe previously discussed1 a variety of WT programs that were developed to treat boiler systems, including precipi-tation (carbonate cycle, phosphate cycle, and coordinated phosphate), chelant, and all polymer. Boiler water treat-ment (BWT) programs incorporate multi-functional DCPs whose functions include sludge conditioning, particulate dispersion, and hardness stabilization. Typical BWT formulations incorporate a variety of addi-tives to achieve scale control, dispersion, and corrosion control objectives.


It is imperative that DCPs retain performance under thermal stress conditions in thermal distillation, geothermal energy recovery, and BWT applications. DCP thermal degradation has been the subject of several recent studies. Masler2 investigated the effects of thermal stress on three polymers [polyacrylic acids (PAAs), polymethacrylic acids (PMAAs), polymaleic acids (PMAs)] commonly used in WT applications. Three parameters were measured before and after thermal stress (5% polymer solutions, pH 10.5, 250 ºC [594 psig], 20 hr): the loss of carboxyl (-COOH) groups, the changes in the average molecular weight (MW), and calcium carbonate (CaCO3) inhibition. Under the conditions employed, PAA, PMAA, and PMA had some structural changes. In terms of MW loss, PMAA lost slightly less than PAA, which lost considerably less than PMA. As CaCO3 inhibitors, PAA and PMAA had minimal performance changes, whereas PMA lost substantial performance. These results suggest that chain scission occurs along with decarboxylation. McNeil and Sadeghi3 investigated thermal degradation of Ca- and Mg-PAA salts, and the results showed some similarities to the behavior of PMAA alkali metal salts. In addition, Lepine and Gilbert4 investigated thermal stress simulating high-pressure heaters and steam generators in nuclear power plants, showing that PAA degradation is complex and involves both decarboxylation and polymer chain integ-rity. This study’s focus is DCP characterization (i.e.,

MW, functional group stability, and acid number) after exposure to varying degrees of thermal stress.

ExperimentalTable 1 lists the DCPs evaluated. The stock DCP solu-tions were prepared in distilled water on an active solids basis. Lubrizol’s protocol to simulate DCP thermal stress in BWT conditions was previously discussed.1

DCP MWs were determined using gel permeation chro-matography (GPC). Nuclear magnetic resonance (NMR) spectra of DCPs before and afterthermal treatment were obtained on a Bruker® AV-500 NMR spectrometer. DCP aqueous solution carboxyl (-COOH) contents before and after thermal stress were determined using standard potassium hydroxide (KOH) by a potentio-metric titration method, and the results are expressed as mg KOH/g polymer (as active solids).

Results and DiscussionAll DCPs undergo changes as a result of thermal stress exposure. These DCP changes depend on thermal stress magnitude and duration, pH, polymer concentration, and DCP composition. Our DCP sample observations 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, copolymer, and terpolymer samples became increasingly turbid and precipitate formation occurred.

Table 1: Polymers Evaluated

Product Composition MW Acronym

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

CK7028* Water polymerized PAA or “WPPAA” 2.3k PAW1

CK732* SPPAA 6k PAS2

CK7058* WPPAA 7.3k PAW2

CPAAP PAA with phosphinate groups <4k PAAP

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

CK766* Sodium polymethacrylate or “PMAA” 5k PMAA1

CK765* PMAA 30k PMAA2

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

<15k CP1

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

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

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

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

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

* Carbosperse™ K-700 polymer and **AMPS® monomer supplied by The Lubrizol Corporation.


Figure 1 shows an AA/SA copolymer structure and possible degrada-tion routes due to thermal stress, as follows:

a. Scission of polymer backbone resulting in different fractions of varying MW

b. Decarboxylation (e.g., –COOH group loss)

c. Hydrolysis of pendent and/or end groups (e.g., ester, amide)

d. Combinations of (a), (b), and/or (c).

Thermal stress-caused hydrolysis in some cases increases the –COOH group content to levels higher than in the original DCP.

Figure 1. Possible DCP Degradation Routes During Thermal Stress

Molecular Weight

Figures 2 and 3 depict DCP GPC MW vs. thermal stress for PAW1 and PMA, respectively. Figure 2 shows relatively small (<3%) MW losses for PAW1, suggesting minimal cleaving of polymer chains, whereas Figure 3 shows a pronounced change in the PMA GPC MW. Comparing Figures 2 and 3 indicates that the PMA’s relative MW loss is greater than for PAW1.

Figure 4 shows CP1 (AA/ SA) MW vs. thermal stress. The rela-tive values (see inset box) indicate that MW losses increase with stress, suggesting some scission of the polymer backbone at ≥200 °C.

Deposit Control Polymer Selection Criteria for High-Temperature Applications. Part 1: Polymer Characterizations continued

Figure 2. PAW1 GPC MW vs. Thermal Stress

Figure 3. PMA GPC MW vs. Thermal Stress

Figure 4. CP1 GPC MW vs. Thermal Stress


Table 2 summarizes the DCP MW losses. Comparing MW losses before and after 250 °C thermal stress for the homopolymers leads to observations that PAA and PMAA MW losses were all <7%, whereas the PMA MW loss was approximately 8%. By comparison, MW losses due to thermal stress for both the SA-based co/terpolymers were >10% at ≥200 °C, with the relative MW losses varying depending upon the initial MW and DCP composition.

Table 2: Polymer MW Loss Caused by Thermal Stress

PolymerMW loss @

150 °CMW loss @

200 °CMW loss @

250 °CPAS1 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%

PMAA1 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%

Decarboxylation

DCP decarboxylation causes -COOH group content loss that decreases DCP inhibitory and/or dispersancy activity and presumably decreases associated efficacy as components of WT programs. Table 3 summarizes the total acid number (TAN) or -COOH content for several homopolymers before and after thermal stress. The data indicate that the PAAs and PMAA1 had insignificant –COOH content changes (e.g., approximately 2% loss for the PAAs) with increasing thermal stress.

Table 3: Homopolymer TAN Values vs. Thermal Stress

Polymer

TAN (mg KOH/g polymer)before heat treatment

(23 oC)

TAN (mg KOH/g polymer)after

heat treatment (250 °C)

% TAN (loss/gain)*

PAS1 606 600 -1

PAW1 620 603 -3

PAS2 638 643 1

PAW2 650 638 -2

PMAA1 623 614 -1

*% TAN (loss/gain) values ±3% are within experimental error

The co- and terpolymer TAN data presented in Table 4 indicate that DCPs containing amide group (i.e., SA) all underwent degradation/hydrolysis resulting in increased TAN values. All DCP post-thermal stress TAN values in Table 4 increased in the range of 6% to 15%, with higher TAN values for the DCPs with greater initial SA content and/or higher initial MW values. Post thermal stress co/terpoly-mers have higher TAN values that result in improved CaCO3 and CaSO4 • 2H2O scale control, but more importantly decreased performance as dispersants and probably as calcium phosphate scale inhibitors.

Table 4: Co/Terpolymer TAN Values vs. Thermal Stress

Polymer

TAN (mg KOH/g polymer) before heat treatment

(23 °C)

TAN (mg KOH/g polymer) after heat treatment

(250 °C)

TAN (% gain)

CP1 508 540 6

CP2 521 598 15

TP1 451 490 9

TP2 479 519 8

TP3 443 496 12

NMR Characterization

Reference proton NMR spectra were obtained for polymer samples before thermal stress. Thermally stressed at 150, 200, and 250 °C, DCP samples prepared in deuterium oxide (D2O) were analyzed directly by using a proton NMR. All samples showed a partial, albeit increasing with temperature, replace-ment of the methyne proton [–CH(COOH)-] back-bone with its deuterated form [–CD(COOH)-]. Figures 5 through 7 present proton NMR spectra vs. thermal stress for PAW1, PMA, and TP1, respectively.

Figure 5 shows the PAW1 backbone methyne (CH) protons and methylene group (CH2) protons appearing at 2.1 mg/L (chemical shift) and at 1.2 to 1.9 mg/L, respectively. In addition, DCP end/head groups appear unchanged at 2.8 to 3.4 mg/L. The lack of new end groups indicates no MW change; an increase in end group type and size relative to the polymeric backbone would have suggested degradation.

The Figure 6 PMA proton NMR spectra obtained before and after thermal stress in D2O agree with the compo-sition above and show the backbone peaks decreasing



during thermal stress mostly due to an alpha-deuteration side reac-tion. The aromatic impurities are expected to undergo a similar reac-tion. The PMA MW reduction (i.e., <1,000 and 50% lower than PAS1 and PAW1 MW values) via polymer chain scission is less obvious by NMR but is indicated by GPC analysis.

The Figure 7 TP1 (AA/SA/SS terpolymer) proton NMR spectra shows a pattern similar to CP1 (AA/SA copolymer). Above 150 °C, the SA co-monomer side chain begins changing, albeit the macromolecule size is unchanged. The SS component is unchanged with increasing temperature, as is the case for the AA component, including the previously described exchange of the methyne proton [– CH(COOH)-] into its deuter-ated form [–CD(COOH)-]. The SA-based TP1 compositional changes are presumably due to sulfonic acid (-SO3H) containing monomer hydrolysis, which is comparable to what we and others have reported for DCPs incorpo-rating other monomers with hydro-lysable functional groups.

SummaryOur observations and conclu-sions based on the laboratory and analytical work discussed herein are summarized below:

1. All polymers, when subjected to the thermal stress conditions (up to 250 °C for 20 hr at pH 10.5), undergo some degradation and/or compositional changes.

2. All polymers tested experience MW changes due to thermal stress.


Figure 5: PAW1 Proton NMR Spectra vs. Thermal Stress

Figure 6: PMA Proton NMR Spectra vs. Thermal Stress

Figure 7: TP1 Proton NMR Spectra vs. Thermal Stress


a. The homopolymer MW losses at 250 °C were all <8% with PMA having the greatest MW loss.

b. The SA-based co/terpolymer MW losses were >11% at ≥200 °C and >12% at 250 °C.

3. Thermal stress impacts on TAN values depend on polymer chemistry.a. The homopolymer TAN values indicate

insignificant –COOH content changes.b. Post thermal stress co/terpolymers have higher

TAN values that result in improved CaCO3 and CaSO4 • 2H2O scale control, but more importantly, decrease performance as dispersants and as calcium phosphate scale inhibitors.

4. Proton NMR characterizations for thermally stressed DCPs indicate:a. PAAs and PMAA showed only slight structural

changes.b. PMA showed significant structural changes that are

not fully understood.c. SA-based co/terpolymers show limited structural

changes at ≤150 °C. However, after thermal stress at ≥200 °C, the SA co-monomer hydrolyzes to acrylic acid.

AcknowledgementsThe authors thank our colleagues for their contribu-tions and The Lubrizol Corporation for permission and support to conduct this study.

References1. Z. Amjad and R. Zuhl, “The Impact of Thermal Stability on the

Performance of Polymeric Dispersants for Boiler Water System,” Association of Water Technologies Annual Convention, Palm Springs, CA, September (2005)

2. W. F. Masler, “Characterization and Thermal Stability of Polymers for Boiler Treatment,” 43rd Annual Meeting, International Water Confer-ence, Pittsburgh, PA, October (1982)

3. C. McNeil and S. M. T. Sadeghi, “Thermal Degradation of Mechanisms of Poly(acrylic acid) and its Salts: Part 3. Magnesium and Calcium Salts,” Polymer Degradation and Stability 30, 267-282 (1990)

4. L. Lepine and R. Gilbert, “Thermal Degradation of Poly(acrylic acid) in Dilute Aqueous Solution,” Polymer Degradation and Stability, 75, 337-345 (2002)

Robert W. Zuhl, P.E., is the global business manager, Water Treatment Chemicals, at The Lubrizol Corporation. He can be reached at (440) 347-7584 or [email protected]. Zahid Amjad, Ph.D., is a technical consultant at The Lubrizol Corporation. He can be reached at (216) 447-5475 or [email protected]. John F. Zibrida is the president of ZIBEX, Inc. and can be reached at (770) 417-1426 or [email protected].

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