Water Soluble Polymers

Wednesday, January 18, 2012

key points

Commercial Uses:

paper making (flocculent)

waste water treatment (flocculent)

oil recovery (viscosity enhancer, flow modifier)

food (gelling agents, creamers, thickeners, binders)

adhesives (paper, cardboard, wallpaper)

pharmaceuticals (suspension agents)

cosmetics (creamers, stabilizers, binders)

textiles (sizers, dye spreaders)

fertilizers (suspension agents for particulates)

biotechnology (culture media)

Polymers used:

Synthetic

polyvinyl alcohol (partially de-esterified PVAc)

polyacrylamides: (-CH2-CH(CONH2)-)n


random copolymer, made by freee radical copolymerization of acrylic acid and acrylamide, or by reaction of homopolyacrylamide with alkali to convert some of the amides to acid groups.

polyacrylic acid (partially esterified)

polyisocyanates

cationic polyelectrolytes

Natural (industrial gums)

starch and its components:

amylase (20-30% in most starches, button extraction)

amylopectin (very high Mwt, branched)

starch derivatives: ethoxy, amino- (cationic)

cellulose derivatives: carboxymethyl, hydroxyethyl, methyl (These derivatives are made by formation of a soda cellulose complex of cellulose (with NaOH) and then treatment with ClCH2COONa, ethylene oxide, or methanol, respectively)

guar gum - from the seeds of the guar plant

locust bean gum

pectins - extracted from peels, e.g orange peel

bacterial polysaccharides: xanthan, scleroglucan, dextrin

chitosan (deacetylated chitin - cationic)

seaweed polysaccharides: agar, alginate, carrageenans, laminarin

Basically water soluble polymers are highly hydrophilic as a result of the presence of oxygen and nitrogen atoms: hydroxyl, carboxylic acid, sulfonate, phosphate, amino, imino groups etc. Their properties in solution are complex: e.g. the solubilities of PAA and polyacrylamide increase with temperature, but others such as PEO, PPO and polymethacrylic acid precipitate at higher temperatures. Where there are ionic groups the solution properties are pH dependent.

Over 4 million tons of polysaccharides are produced annually for use as industrial gums. 75% of this total is starch, of which the major application is adhesives.    The ability of polysaccharides to crystallize is important, meaning that there will be strong intermolecular bonding the solid state, and at the tie points in gels. Branching enhances solubility, e.g. amylopectin versus amylose, probably because the branched molecules are more globular and there is less opportunity for entanglements in solution. Charged polysaccharides such as pectin and carrageenans have expanded conformations in solution as a result of intra-residue repulsions, and the intermolecular repulsions lead to stable solutions. Divalent cations can bridge between the chains leading to tie points and gel formation.   The chemical structures of some of these polysaccharides are shown below.

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Agar, carrageenan and alginate are all seaweed polysaccharides used as gelling agents in the food industry.   In gels the polymers form a network in which the cross links are non-covalent, and hence are reversible so that the gel can be converted to a solution by a change in temperature, pH or ionic strength.   Agar is used by the biochemists a a gel for culturing bacteria, and it is the basis of many food products.    The high molecular weight polymer yields a strong gel at only 1% concentration. It is a neutral (uncharged) repeating disaccharide of beta 1,3 galactose and beta 1,4 anhydroglucose.   Carrageenan is a family of similar polydisaccharides, of which the kappa form is one example: alternating beta 1,3-D galactose 4-sulfate and beta 1,4-anhydro-D-glucose, so it is a polyanion, and its gelation is particularly sensitive to the presence of divalent ions, such as Ca2+, which bridge between the sulfate groups of adjacent chains.   Alginate is an example of a natural block copolymer.   There are two acidic monomers, beta-1,4 manuronate (M) and alpha-1,4-guluronate (G), and depending on the type of seaweed, there are different proportions of (M)n and G)n blocks, as well as some random G/M regions.    The homopolymer blocks differ in stiffness, and tie-points are formed between the identical blocks, notably (G)n, via divalent ions.

Xanthan (above) is an extracellular polysaccharide produced by the bacterium Xanthamonas campestris.   The polymer is produced by industrial fermentation: perhaps the bacterium produces it as some sort of defense mechanism.    The polymer is a polypentasaccharide: a cellulose backbone with a tri-saccharide sidechain on every second backbone glucose unit.   The second sugar of the sidechain is anionic, and there is further charge by addition of pyruvate units to some of the ends of the sidechain.   The molecular weight is about 5 million.   The backbone is stiff, and the stiffness is further enhanced by the formation of a double helical structure.    This leads to high viscosity, so that low concentrations can be used as viscosity enhancers.

USDA regulations require that food processors put a list of ingredients on their packaging.   These lists can make interesting reading for polymer scientists.   A label from a supermarket ice-cream carton is shown below.    Tastes vary - this was some sort of almond cookie mixture!   Note that the ingredients are in order of their proportions, so one is relieved to find dairy products lead the list.   Pectin is in there as a gelling agent in the fudge ribbon, which is essentially a separate phase.   Cellulose gum (a derivative, probably carboxymethylcellulose), guar gum and carrageenan are viscosity enhancers and gelling agents

icecream.JPG (74132 bytes)

Link to a very interesting web page from U. Guelph on the history and methods of ice cream making

Papermaking

Paper is made from a slurry of wood cellulose fibers (pulp) which is spread into a web, drained, pressed between felts and then dried on heated drums. Water soluble polymers are added to the beaten pulp and serve as flocculent that aid the adhesion of the fibers to one another. They can do this by balancing the charges and bridging the fibers. Paper fibers tend to be anionic due to the presence of uronic acid residues in the hemicellulose component, and additional negatively charged groups are produced during pulping. A cationic polymer interacts with these charged groups and effectively breaks down their hydration shells, leading to better drainage.

The water soluble polymers in paper making also improve the retention of fillers and other added particulates such as pigments, especially TiO2. TiO2 is expensive and it is necessary to avoid waste, and also to avoid TiO2 pollution in waste water. The TiO2 particles are effectively bonded to the fibers by the added water soluble polymer, leading to better distribution, so that less TiO2 is needed. In the absence of flocculant the TiO2 particles tend to agglomerate and plug holes in the web, adding to the drainage problem.

The polymers used in paper making include polyacrylamides, polyvinyl alcohol, polyamines and cationic starches (especially amino-starches).

polyacrylamide: 0.01-0.05% dry weight for TiO2 retention

0.03-0.02% for better drainage

cationic starch 0.25-0.75% dry weight also better strength

Superabsorbant Polymers     Erosion Control

Waste Water Treatment

Typical waste water contains sewage, industrial wastes and storm water.    Primary treatment is via sedimentation and aeration, which removes ~50% of organic wastes.  Secondary treatment involves further aeration and sedimentation. Starting in 1900 lime was added, along with Al and Fe salts, to assist in agglomeration of suspended materials by charge balancing and bridging. From 1950 onwards the inorganic additives (themselves pollutants) have been replaces by macromolecular flocculants. Note that addition of these polymers (usually polyelectrolytes) to waste water can have significant advantages in terms of drag reduction.

Polyanions are used to precipitate organic wastes and also to remove phosphates.    These include

Polyacrylates

Copoly(vinyl alcohol/acrylate)

poly(acrylamide/acrylic acid)

poly(acrylonitrile/acrylic acid)

polystyrene sulfonate, polyethylene sulfonate

lignin sulfonate - pulping byproduct

Quantities 0.2-1.0 mg/liter (0.0001%) results in approximately twice the waste removal

The residue is in the form of sludge, with a relatively high water content. and needs to be dried. Cationic polymers are added at this stage and interact with the negatively charged particles, leading to further agglomeration, breakage of hydration shells, and hence easier drying.   Cationic starch derivatives and chitosan are used, and also copolymers of aminoalkyl-acrylamides and methacrylamides, such as CH2=CHCOO(CH2)2N+(CH2)2Cl+, which is a quaternary ionic amine.   A positively charged melamine/formaldehyde copolymer can be made water soluble (and positively charged) by reaction in the presence of HCl.   This polycation is cheap and utilized extensively in waste water treatment (and in paper making) but the formaldehyde presents an environmental problem.    Cationic polymers produced by reaction of dicyandiamides with formaldehyde are used to remove dyes and inks from waste water.

Oil Recovery

Water soluble polymers have been routinely used by the oil industry as lubricants in drilling muds.   They are also used in dilute solution in water pumped in to oil wells to enhance recovery.   Oil is trapped in porous rock, and water is pumped in to wells this out.   The process is "fracking".

Primary recovery: oil is extracted from the well under the pressure of natural gas, or by pumping, by which some 15-20% of the total oil (in the ground) is obtained    The pressure of natural gas may be as high as 15000psi, and the temperature can be as high as 150degC.   Also there is a lot of water - perhaps as much as 7 barrels per barrel of oil.

Secondary recovery: water is pumped in at separate injection wells in the neighborhood. An additional 10-20% of the oil may be removed at this stage, as globules suspended in the exiting water. This process ceases to be economic when the water to oil ratio exceeds 20 to 1(depending of course on the price of oil). The water is forced through the porous rock strata. Steam may be used where the oil is very viscous.

Tertiary recovery - fracking   The yield during water pumping is increased by addition of water soluble polymer, which increases the viscosity. This process was first used by Shell in the 1940s.   The mechanism is not fully understood, but it is thought to depend on the increased viscosity of the dilute polymer solution. The more viscous pumping solution is less likely to "finger" through and past the oil-water interface, so that it progressively pushes the oil ahead of it. Passage of the solution through the narrow pores or capillaries is still possible because of shear thinning effects.    The presence of the polymer improves the sweep efficiency by slowing passage through regions that are more highly porous, making it more likely that at lease some of the water will go into otherwise bypassed regions and bring out the oil.    Very lightly crosslinked polymers (acrylamides) are also of interest here in that the particles tend to block the high flow regions.

We need solubility and hydrolytic stability in a relatively hostile environment - temperature, and NaCl (brine).   To achieve high viscosity at low concentration we need high molecular weight, and a stiff conformation is helpful.   we need shear stability, in that the solution will be forced through capillaries and so the chains will be elongated.   The polymer needs to be cheap because large volumes of water will be used, and it should be non-toxic because the water needs to be disposed of eventually.   The polymers used include polyacrylamide and its copolymers, vinyl alcohol copolymers, carboxymethyl cellulose, guar and xanthan. The acrylamides are cheap synthetics with high Mwt - 10^6, high solution viscosity, but they are subject to shear degradation and tend to be precipitated by divalent ions.   Carboxymethyl cellulose is cheap, and a 0.06-0.08% solution results in a 3-4x increase in viscosity (over untreated water). Polysaccharides are less susceptible to degradation at higher temperatures than the polyacrylamides.   Xanthan has a stiffer conformation, and a higher molecular weight, but is x10 more expensive, which is a problem even though much lower concentrations can be used (as compared to CMC and polyacrylamides).   The economics of tertiary oil recovery only make sense when oil costs somewhere in the region of $30-40/barrel using polymers that cost about $3/lb.   Interest faded with cheaper prices after the high in the late 1970s when oil was about $10 per barrel, but we are way above that now - hence the current interest in fracking.

More details on the use of guar  Structure of guar moleculehttp://www.chemtotal.com/guar-gum.html

 

 

 

 

 

 

 

 

 

Other applications:

Adhesives: Starch, collagen, casein

Foods: gelling agents in ice creams, puddings, whipped desserts (carboxymethyl cellulose, carrageenans, guar gum), creamers in cottage cheese (carrageenan, xanthan), binders in ground meat and in spun protein fibers (xanthan, carrageenan), beverages: viscosity enhancers, foam heads (xanthan, alginates), jams and jellies (pectin), salad dressing thickeners (xanthan, guar), sauce thickeners (starch)

Pharmaceuticals: suspension agents for insoluble drugs, kaolin, oils, herbicides, insecticides (pectin, carrageenan)

Cosmetics: texture agents (polyvinyl alcohol, locust bean gum, guar, carrageenan)

Hair care products: in shampoos and conditioners, as thickeners, opacifiers, to increase body/fullness/gloss.   hair is negatively charged, and cationic polymers are included in shampoo formulations, forming a layer of aggregated polymer around the hair which is not removed on rinsing.   Favorites are quaternary amine derivatives of cellulose.   Silicone polymers may also be included, which have low surface tension and easily wet the surface of hairs.    Cellulose derivatives, especially hydroxyproply cellulose, are added to enhance lathering, and dispersions of vinylpyrolidone copolymers are added as opacifiers.    Polyvinylpyrolidone has been used in hair sprays since the 1940s.    (Historically shellac was used for centuries.   Shellac is predominantly a long-chain hydroxycarbonic acid secreted by an insect, Lacifer ilecea, native to China.)   Polyvinypyrolidone has the problem that it is too hygroscopic, and in the 1950s this was replaced by copolymers of vinylpyrolidone with vinyl acetate.   These polymers form a thin transparent layer on hair, adding stiffness, and (unlike shellac) are easily removed.   With mousses a gelling agent is also added.

Paints: creamers in latex paint hydroxyethylcellulose, xanthan

Textiles: sizing agents, dye spreaders (starch, polyvinyl alcohol, xanthan)

C&ENews - Soaps and Detergents 2005