Superabsorbent Polymers as Water-Blocking Components in Cables

Transcription

1 Superabsorbent Polymers as Water-Blocking Components in Cables 1) Introduction Superabsorbent polymers (SAP's) have been successfully marketed with consistently increasing success since the 1980's. This duroplastic synthetic material can take in many times its own weight in water (absorption), forming a so-called "hydrogel", and hold this water tightly even under applied pressure (retention). The main end use is for hygiene articles, like diapers, incontinence articles and feminine hygiene articles. In addition, a technically oriented market has evolved that since the 1990's has developed ever more technical application fields. Examples include products to retain soil moisture, "soil conditioners", for packaging applications and to prevent water infiltration in cables. This last application, due to its complex requirements, requires an especially detailed examination. 1) Modern Superabsorbent Polymer (SAP) High performance SAP is synthesized as a cross-linked sodium salt of acrylic acid, although other raw materials are possible. Taylor-made application-specific polymers can be produced through appropriate modifications. Through a combination of careful control of cross-linker type and amount, as well as reaction conditions, today's market includes SAP's that not only have high water absorption and storage capacity, but also targeted permeability, stability and rheological properties. 2) Polymerization and Crosslinking 2.1 Polymerization SAP based on crosslinked acrylic acid sodium salt is synthesized by a radical chain polymerization in aqueous solution. The chain formation and crosslinking steps take place in the same medium, resulting in a network with embedded reaction residues comprising non-crosslinked oligomers and polymers. These components are water soluble and make up the so called "extractables". Due to their molecular weight, these extractables have a viscosifying or thickening effect on water. Two important technical procedures will differ. On the one hand a Susoension polymerization process where the monomer is finely dispersed in a nonpolar solvent. These micelles form spherical SAP particles that are deposited from the solvent and processed by agglomeration. Another manufacturing process is the bulk polymerization process, in which the monomer solution is pumped onto a conveyor belt or a kneader where polymerization takes place. This produces a compact gel block which is crushed and milled. This process produces irregular, crystal-like particles. 2.2) Crosslinker Generally, all compounds with at least two polymerizable double bonds, one polymerizable double bond and a functional group capable of reacting with the monomer, or two monomer-reactive functional groups, can react as crosslinkers. Additionally, multivalent cations are possible. Preferred crosslinkers, for example,

2 are N,N-bisacrylamide, triallylamine, (poly)acrylic acid esters from polyols or (meth)allyl esters. 2.3) Polymerization and Product Properties The most important application-specific properties of an SAP as a blockling agent are: - Absorption and retention - Absorption speed - Water conductivity (permeability) - Stability against hydroylsis/gel breakdown These product properties are achieved by a suitable reaction procedure, careful selection of the crosslinker system, specific additives, a defined particle size and surface treatments ) Absorption and Retention Both properties are determined by the structure of the network. An open, lightly crosslinked polymer results in a high absorption and retention capacity. A more strongly crosslinked polymer has correspondingly lower values. Retention / Absorption Degree of crosslinking (crosslinker/ monomer ratio) 2.3.2) Absorption speed The absorption speed is determined by the polymer structure and the contact surface area between the SAP and water. Therefore, a small particle size is advantageous. Another important attributes is the elastic modulus of the SAP. During the absorption process the swelling power must work against the restoring force of the polymer, which can be construed as elasticity of the polymer.[0]

3 2.3.3) Permeability Permeability refers to the capability of swollen hydrogels to transport and distribute liquids. This process occurs through capillary transport through the pores and interstitial spaces between the gel particles. The transport of liquid through the swollen superabsorbent gel particles themselves follows the laws of diffusion and is a very slow process. Two important factors in the porosity of an SAP are the elastic modulus of the hydrogel and the particle size. The porosity increases with the square of the particle diameter. ) [0]. A narrow particle size distribution, or monomodal particle size also favors higher porosity. Packing density is increased and porosity reduced with multi-modal particle size, and with an irregular particle shape ) Gelstrength/Elastic Modulus The stability of the network against shear stress is function of the degree of crosslinking. The higher the number of cross-link in the network is the higher is the modulus.

4 PSD: Monomodal or Multimodal Shape: Round or irregular Likewise, the porosity increases with the Elastic Modulus [0]. Hydrogels with a high modulus have high gel strengths and are more difficult to deform and reduce the interstitial gaps. They therefore maintain a more open gel bed structure and possess a greater porosity compared to easily deformed particles with low gel strength.

5 Therefore, SAP's that are suitable for sealing or blocking materials should have the smallest possible particle size and exhibit a low elastic modulus. By observing the packing density of hydrogels, it is also clear that a mult-modal particle size distribution, as well as irregularly shaped particles are advantageous ) Extractables The extractables in a SAP are short or long chain, uncrosslinked (acrylate) polymers and, therefore, are water soluble. When excess water is present, they slowly work themselves out of the crosslinked network, dissolving into the water phase. There they act as all high molecular weight polyelectrolytes in water do, as thickeners. In accordance with Darcy's Law, the penetration speed of a fluid through a porous medium decreases with the viscosity of the fluid [0]. Therefore, a sufficient amount of extractable polymer is also advantageous for an SAP s effectiveness as a water-blocking compound. 3) Special SAP for Cable Applications The previously-named properties already show that there are a wealth of criteria required for a SAP to be suitable as a water-blocking agent in cables. In addition to the obvious main responsibility - to stop a water infiltration as quickly as possible within the shortest distance of the break-in point - the longevity of the hydrogel plays an important role, as cables in such applications have a planned operational lifetime of 40 years or more. Gourman/Taupin [2] und Gruhn [3] have mentioned this aspect and Clyburn has described suitable test methods to determine the hydrogel's stability and resistance to hydrolysis. However, it was not yet clear exactly what influence progressive hydrolysis had on the blocking performance. Following, a test design will be described where the short-term and long-term penetration behavior of SAP in small diameter capillaries can be

6 determined. The tests were conducted at different temperatures with different types of waters. 3.1) SAP's tested and performance data Sample Code Morphology Bead Bead Bead Crystal Crystal Crystal Absorption in DI Water [g/g] Absorption Speed [mm after 1 min.] > 16 > ) Hydrolysis Resistance and Gel Degradation When in contact with water, SAP's expand their volume and absorb large amounts of water, which they are able to retain even under pressure. However, the hydrogels age differently, according to their individual polymer characteristics. Products that are crosslinked with esters, fluidize after a relatively short time because the ester groups hydrolyze and the formerly crosslinked, water insoluble polymer degrades to a linear, now water soluble polymer. Somewhat more stable are the nitrogen-containing crosslinkers, that can also be degraded by an oxidative process on the nitrogen. All SAP's together can also be broken down by either a metal-catalyzed or exothermic degredation of the carbon backbone, which reduces the absorption characteristics and eventually destroys the hydrogel. These degradations can be determined through measuring the viscosity of the hydrogel slurry.because the water bond in the network gets free again and dilute the slurry. The more the hydrolysis process proceeds the more water is set free and reduces the viscosity of the slurry. For this purpose, samples of SAP are swollen to 50% of their maximum absorption capacity and the time dependent viscosity of this slurry is recorded with a Brookfield Viscometer (RV-I). [4] Whereby the initial viscosity is set as 100% and the following values are calculated as percentage. To accelerate the degradation, slurries are stored in darkness at 80 C. Dramatic differences can be seen, as shown in the graph.

7 Degradation of hydrogel made with DI-water and stored at 80 C (x-axis not linear)

8 Degradation of hydrogel made with Synthetic Sea Water (DIN 50900) and stored at 80 C (x-axis not linear)

9 Degradation of hydrogel made with hard tap-water (Grade4) and stored at 80 C (x-axis not linear) The found results show that a lot of existing cable test-methodes do not allow to describe the reality exactly. The cable SAP were tested either in a dry status at higher temperatures e.g. at 90 C or in a hydrolised status at 20 C for a shorter time e.g. 1 day. The here shown data indicate that a longer time of testing the hydrolised SAP at higher temperatures is of importance to get information about the longterm stability. Hvidsten [6] showed that the impact of free water in a cable what is watertrees and corrosion. Only a long-term hydrolysis-stable SAP can prevent those negative effects as free water is bond effective and log lastin in the SAP network. If that network collapses the water gets free and can start its work of destruction. Both the two analytical methods shown here are able to create information about the long-term sealing property. Either indirect by watching the drop in gel-viscosity or direct by measuring the penetration length under different temperaturs. 3.3) Rheological Properties As the rheological properties of swollen SAP have a significant influence on the permeability and thereby the blocking ability, how the behavior of the hydrogel s elasticity during the swelling process and what elasticity the final swollen gels attain was investigated. Thereto, gels were synthesized and swollen to 25%, 50% and 100% of their maximum absorption. The elastic modulus of these gels was then determined with a plate rheometer.

10 12 Viscosity/Gelstrength [[logpa] Quarter Saturated Half Saturated Full Saturated 0 #1 #2 #3 #4 #5 #6 Sample Correlation Gelviscosity/Gelstrength and Degree of saturation (DI-water) To better illustrate the values found in fully saturated condition, the values in the graphic have been converted to Pascals [Pa].

11 7 6 5 Gelstrength [Pa] #1 #2 #3 #4 #5 #6 Sample Gelstrength of 100% saturated (DI-water) SAP 3.3) Permeability Determination Gruhn described the open spaces in cables as being from 0.2 to 0.8 mm² [3]. Commercially available melting point capillaries have an internal clearance from 2mm corresponding to 0.31 mm² and thereby are well suited as a model for cable capillaries, even if the hydraulic properties in detail differ. Such capillaries were filled with SAP powder. Through calibration of the fill level and determination of the fill weight, the test samples were set at a uniform SAP powder packing density of (380 ± 20) g/l. In this way, the influence of different packing densities could be eliminated. The construction of the test apparatus was adopted from what is described in various standards. The test sample is connected to a 1m water column and the penetration of the liquid front in the longitudinal direction is observed.

12 Shematic Set-up of Test Equioment The penetration length in mm after a defined time is recorded as the key parameter. The test is done at room-temperature (20 C) as well as at 60 C in a thermo Chamber (see photo). Picture of Test-Equipment in Thermo Chamber

13 3.4) Test Method An amount of SAP is placed on a watch glass to reach a fill height of approximately 5 mm. An appropriate melting point determination capillary (D= 2mm, L = 120 mm, ex. Hirschmann Artikel ) is inverted and pressed down onto the SAP in the watch glass multiple times until the capillary is 5 mm filled. Then the partially-filled capillary tube is righted (closed end down) and dropped from a 10 cm height 3 times onto a hard surface (such as a table top) so that the SAP in compacted at the closed end of the tube. This process is repeated until the capillary is filled to within 10 mm of the open end. A small amount of cotton batting is then pressed into the capillary to seal the tube and hold the SAP in place. The fill height and SAP fill weight (mass of filled capillary - empty capillary) is measured and recorded. The weight of the cotton batting can be neglected. The packing density can be calculated from these two values (mass of SAP [g]/fill height [cm] = packing density [g/cm]) The packing density between different samples should vary maximum +/- 5% so that the results are comparable. The closed end of the tube is now cut off immediately above the seal, to create a filled tube of SAP with 2 open ends, allowing incoming fluid to push air out the other side of the tube. The now open end is closed with cotton batting by pushing it into with a needle. The capillary is now placed into the measurement apparatus in a manner that does not allow leakage. (the use of Parafilm to wrap the connection point is recommended) The SAP/cotton contact surface serves as the starting point and is marked with a fine felt pen. How far the fluid has penetrated into the SAP is now measured at predetermined times. This is recorded as the distance between the fine marker line originally placed at the cotton batting and the border between swollen and dry SAP. This border is recognizable as the point the SAP changes from transparent/milky to opaque/white. (Photo) Built in Cappilary

14 3.4.) Results The data created by the described method are shown in the following tables. The values are in mm. Picture of a treated capillary. The red lines indicates the measured Penetration distance ) DI Water at 20 C Time Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Sample 6 Minutes Days Density [g/g]

15 3.4.2) DI-Water at 60 C Time Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Sample 6 Minutes Days Density [g/g] ) Tap Water (Hardness 4) at 20 C Time Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Sample 6 Minutes Days Density [g/g]

16 3.4.4) Tap Water (Hardness 4) at 60 C Time Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Sample 6 Minutes Days Density [g/g] ) Synthetic Seawater (DIN 50900) at 20 C Time Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Sample 6 Minutes Days Density [g/g]

17 3.4.6) Synthetic Seawater (DIN 50900) at 60 C Time Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Sample 6 Minutes Days Density [g/g] ) Discussion of Results The values show that the ability of a SAP to retain the penetrating water is dependent on the temperature as well as the composition of the water. The amount and type of dissolved salts interferes with the swelling properties of the polymers and their hydrolytic degradation. Higher temperatures appear to be initially positive for the sealing whose only penetration distances at 60 C in the first few hours are shorter than at 20. This is most probably due to an increase in swell rate due to increase temperature. Long term, the high temperature, however, has a disadvantage with stablity because of the hydrolysis process is also accelerated due to the higher reaction rate at higher temperatures. Interesting was the influence of salts on stability. The total reduced absorption capability of SAP for saline waters could also expect a significant deterioration in sealing performance. That this is not the case, some even better values were measured, is explained by the formation of additional cross-linking bridges with multivalent cations. This will counteract the hydrolytic degradation and stabilizes the hydrogel. This effect is already indicated in the investigation of the viscosity reduction. Compared to gels of DI water, the degradation curves after the first few days are somewhat flattened, which is due to the stabilizing influence of the cations. An exception is # 6. This product degrades faster in saline water than in deionized water, but in all cases it is the most stable. This explains that this product delivers in all the best penetration test values. Finally, the data modulus of elasticity and penetration length at 20 C in DI water was correlated. It turns out that in accordance with the physical models for the penetration, the polymers with lower modulus have the shortest penetration lengths. Further work to investigate the temperature and salt dependence in more detail are still required.

18 Correlation E-Modulud and Penetrationlength (DI-Water at 20 C] Conclusion The current body of material specifications, as well as performance standards for dry water blocked power cables, focus on the rapid swelling reaction and high absorbtion capacity of SAP in the dry state. Already Clyburn [4] has pointed out that the long term stability of the hydrated SAP is critical to water penetration performance over the life cycle of a cable. The research presented in this paper reinforces the importance of that property, and also points to the importance of the type of cross-linking agent and the modulus of elasticity of the SAP. It is significant that the test data shows that SAP designed to optimize these properties exhibit much improved cable penetration results, compared to SAP designed for faster swelling/high absorbency. The present work has shown that the difference in performance is magnified at higher temperatures, such as are inherent in energized power cables. With the proposed new type of penetration test, it is possible to predict the behavior in a short time of a swollen SAP at different temperatures and at using different water qualities.

19 Through the selection of suitable crosslinking agents, particle morphology, modulus of elasticity, and resistance to hydrolysis, it is possible to produce SAP with the optimal blend of properties designed for the life cycle of the cable. Perhaps this suggests that existing water penetration tests for cables should be modified to incorporate analysis of these important factors. Expression of thanks: The author thanks Ms Nadine Bartels, Evonik Industries, for doing all the tests and measurements; Dr. Scott Smith, Mr. Bobby Mitra and Mr. Alton Deaton, Evonik Industries, for the assistance in translate the article; Dr. Jochen Houben, Ashalnd Deutschland GmbH for the discussions and assistance. Literatur: [0] L. Buchholz et al., Modern Superabsorbent Polymer Technology, Wiley-VCH Verlag GmbH & Co. KG, (1998) [1] M. Frank, Superabsorbents, Ullmann s Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag Gmbh&Co. KG, (2012) [2] M. Gourmand und Y. Taupin, Superabsorbent Polymers (SAP) for Cable Application: Influence of Powder Characteristics on Water-Blocking Properties, Index 99, (1999) [3] J.Gruhn, Characterizing and Selection Superabsorbent Cable Components, IWCS Proceedings pages , (1998) [4] C.E. Clyburn III, Long-Term Stability of Superabsorbent Gel for Cable Waterblocking Performance, 59th IWCS Proceeding pages , (2010) [5] H.-G. Elias, Makromoleküle Band1, Chemische Struktur und Synthesen, Seite 565 ff, Wiley-VCH Verlag GmbH&Co. KG, (1999) [6] Hvidsten et al., Initation of Vented Water Trees by Severe Degradation of the Conductor Screen of Laboratory Aged MV XLPE Insulated Cables, ICC Fall 2011 Meeting (Oct )