Polysulphides

Polysulfides : Polysulfides TC 565 – Mohamad Widodo Spring 2008

Background : Chemists in Switzerland reported a rubbery, intractable, high sulfur semisolid from 1,2-dichloroethane and potassium polysulfide. 1838 Joseph C. Patrick (accidentally) discovered the first polysulfide elastomer. 1924 First commercial polysulfide elastomer was produced by Thiokol Chemical Corp. (USA): Thiokol A (ethylene tetrasulfide). 1929 Background A class of material with alternating chains of several sulfur atoms and hydrocarbons : [–R–Sx–]n R = alkyl, alkoxy x = commonly 2 – 4 (rank) n = 1000 – 4000 or 50,000 –100,000 Definition Development Thiokol has become generic name for polysulfides, just as Nylon for polyamide. TC565 Page 2

Examples of Polysulfides : Examples of Polysulfides TC565 Page 3

Manufacture & Processes : Manufacture & Processes Na2Sx (x = 2 – 6). Equilibrium mixtures of anions in solution, S2- to S62- : Raw Material Sodium polysulphides Prepared by treating sulfur with sodium hydroxide: in slight excess side reactions neutralization TC565 Page 4

Properties of Sodium Polysulfides (Na2S4 ) : Properties of Sodium Polysulfides (Na2S4 ) Dark red somewhat viscous liquid Faint rotten egg odour bp: 113oC Completely soluble in water pH: 12.5 – 13.5 Releases toxic and flammable H2S gas if exposed to excessive heat or contact with acids Corrosive Stable compound SKIN CONTACT: causes irritation, severe burn, absorbed throughout skin leading to H2S poisoning. EYE CONTACT: can cause irreversible damage with possible blindness. INHALATION: hazardous only as a mist unless product is decomposed to hydrogen sulfide which is irritating to the mucous membranes, eyes and respiratory tract and potentially lethal. INGESTION: gastric acids convert product to potentially lethal hydrogen sulfide. Physical & Chemical Characteristics Hazardous Properties Source: http://www.bestsulfurproducts.com/msds/tetragard.pdf TC565 Page 5

Slide 6 : Why do we have to neutralize the unreacted sodium hydroxide? What if we just leave it? Question TC565 To minimize hydrolysis of chloride in the subsequent preparation of polysulfide polymer Page 6

Manufacture : Manufacture Raw Material Alkyl dichlorides Remember me? Used originally in the first commercial production of polysulphide polymers. Giving products with objectionable odour during processing. At present, it is used only for preparation of elastomers with high sulphur content. Used in 90% of current production of polysulphides. Gives elastomers with good all-around properties. Little odour To introduce branching and crosslinking TC565 Page 7

Preparation of bis(2-chloroethyl) formal : Preparation of bis(2-chloroethyl) formal Colorless liquid normally used as solvent. bp. 217.5oC Not flammable Slightly soluble in water Toxic by inhalation and ingestion Strong irritant to skin, eyes, and mucous membranes. Properties http://cameochemicals.noaa.gov/chemical/17082 TC565 Page 8

1,2,3-trichloropropane : 1,2,3-trichloropropane http://www.atsdr.cdc.gov/toxprofiles/tp57-c3.pdf A solvent and extractive agent Sensitive to exposure to light and heat Inhalation of vapor causes anesthesia, dizziness, and nausea. Vapor is highly irritating by inhalation routes and moderately irritating by dermal routes. Exposure of eyes to vapor may result in slight, transient injury to the cornea. TC565 Page 9

Preparation of Polysulfide Polymer & Their Reactions : Preparation of Polysulfide Polymer & Their Reactions (2) Alkyl aryl sulphonate NaOH MgCl2 Na2Sx (20% solid) (1) 80oC (3) Bis(2-chloroethyl) formal 2 hrs MgOH (nucleating agent) cooling < 90oC evolves heat (4) 20% excess Na2Sx (toughening) Polymerization process The resulting dispersion of polymer is then washed with hot water Reactions Stoichiometric deficiency of about 20% is required to assist greater utilization of alkyl chloride : This is how the polymerization reaction takes place : NOTE It represents ideal case only, because polysulfide anions of varying ranks exist and involved in the reaction. Polymer contains a distribution of polysulfide groups Page 10

Concurrent & Other Reactions : Concurrent & Other Reactions TC565 Growing chain attacks dichloride Hydrolysis of chlor-terminated polysulfide (1) by OH- ions Interchange reactions : between polysulfide anions (from sodium polysulfide) with polysulfide groups in polysulfide polymer : (i) not containing OH terminal, and (ii) near OH terminal. between anion terminal in polysulfide polymer and polysulfide groups in another (iii and iv). Hint: It has something to do with equilibrium. Page 11

Note: : TC565 Note: Washing removes the soluble hydroxyl-terminated fragments (low molecular weight polymers) and also inorganic polysulfide anions from the system. As a result the polymer becomes more completely mercaptide-terminated and more increasingly able to participate in the equilibrium shown in 3a(i). The removal of inorganic polysulfide by washing drives the equilibrium in 3a(i) to the left, i.e. higher average MW of polymer. Repeated treatment with fresh polysulfide solution leads to higher MW. Generally, MW of 500,000 is readily achieved. Polymerization proceeds by way of interchange reactions. Equimolar amounts of Na2Sx and dihalide gives only low MW due to competitive reaction with OH- (ref. reaction 2). Excess of polysulfide anions is required to eliminate hydroxyl-terminated polymers … see reaction 3a(ii). Polymerization of polysulfide differs from conventional condensation in that a stoichiometric excess of one of the reactants is used to obtain a high MW. Page 12

Product Types : TC565 Page 13 Product Types

Solid Polymers : TC565 Page 14 Solid Polymers Hydroxyl-terminated polymers Results from hydrolysis of chloride terminals. Aqueous polymer dispersion is coagulated with sulfuric acid; resultant crumb is washed and dried. Its use is limited to applications where outstanding solvent resistance is required. Because of its high MW, OH-terminated polymers are difficult to process: requires addition of ________ , e.g. benzothiazyl disulfide (mercapto-benzothiazole disulfide) plasticizer + H2SO4 Coagulated washed, dried

Solid Polymers : TC565 Page 15 Solid Polymers Thiol-terminated polymers Aqueous dispersion is heated at 80oC for 1 hour with a mixture of NaHS and Na2SO3 Product is washed by decantation The regeneration of polymer is prevented by the presence of Na2SO3, which abstracts sulfur atom from thiothiol terminal to produce thiol. Na2SO3 also removes high rank sulfur in the polysulfide polymer to give disulfide groups. Coagulation by acidification; mercaptide terminals are converted to thiol groups. Increasing the proportion of NaHS and Na2SO3 leads to lower molecular weight of thiol-terminated polymer, thus the production of liquid polymers.

Thiol-terminated Polymers: Curing : TC565 Thiol-terminated Polymers: Curing Curing is performed by oxidative coupling ?SH + [O] + HS? ? ?S?S? + H2O SOLID: The standard curing system is p-quinone dioxime LIQUID: Lead peroxide is most used. ?R?SH + PbO2 + HS?R? ? ?R?S?S?R? + PbO + H2O (1) ?R?SH + PbO + HS?R? ? ?R?S?Pb?S?R? + H2O (2) Reaction (2) is unwanted side-reaction because lead mercaptide links readily participated in interchange reactions with polysulfide groups. This results in products with poor stress relaxation properties. Inclusion of sulfur in the system reduces the number of these links by the formation of lead sulfide: ?R?S?Pb?S?R? + S ? ?R?S?S?R? + PbS (3) Page 16

Slide 17 : An important difference between solid hydroxyl- and thiol-terminated polymers is that the latter are of relatively low molecular weight. Including a trihalide such as 1,2,3-trichloropropane to the system leads to the formation of branched and crosslinked polymer. TC565 Page 17 Question Which one gives result to an easier to handle and less tough material to be worked on ?

Epoxy-modified Polysulfides : TC565 Epoxy-modified Polysulfides Page 18 Liquid thiol-terminated polymers are frequently used in conjuction with epoxy resins, which leads to chain extension: Reaction is accelerated by tertiary amines by forming mercaptide ions (1) or opening of epoxy ring (2) Tertiary amines also catalyze interaction between hydroxyl groups formed and remaining epoxy groups leading to the build-up of crosslinked structure. Epoxy-polysulfide compositions have good impact strength and find use in adhesives and castings.

Effects of Interchange Reactions on Properties of Polysulfide : TC565 Effects of Interchange Reactions on Properties of Polysulfide Page 19 Polysulfide polymers, especially with high sulfur content, has poor compression set resistance, which is due interchange reactions at polysulfide bonds through radical and ionic mechanisms. Radical mechanism Ionic mechanism may occur at room temp.

Effects of Interchange Reactions on Properties of Polysulfide : TC565 Effects of Interchange Reactions on Properties of Polysulfide Page 20 Polysulfide elastomers have good low temperature flexibility, which is determined largely by the length of the chain between polysulfide linkages. Q: In this regard, which one has better low temperature flexibility? Products based on ethylene dichloride or bis(2-chloroethyl) formal? Sulfur content has an effect on the thermal stability of elastomers. Products of higher sulfur content have relatively lower thermal stability. Q: Why?

Properties & Uses : Properties & Uses USES. For sealing joints on airfield runways, parking aprons and other areas where joints may be subject to fuel spillage and jet blast. For use in sealing reservoirs, dams, canals culverts, and flood control channels. Also used for sealing curtain-wall panels, tilt-up panels, window glazing, flashing and metal setting. Also, for all joint-seal applications, such as expansion and contraction joints in shopping centers, sidewalks or other traffic areas. TC565 PROPERTIES Outstanding resistance to oils and solvents. The resistance improves with sulfur content in the polymer. Products of high sulfur content impermeable to vapors and gases. Oxygen and ozone resistant. Good low temperature flexibility. Poor compression set resistance, especially those with high sulfur content. Low tensile strength and abrasion resistance. Page 21