Plasticizer

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Plasticizers are additives that soften the materials (usually a plastic or a concrete mix) they are added to. Although the same compounds are often used for both plastics and concretes, the desired effect is slightly different. The plasticizers for plastics soften the final product increasing its flexibility. Plasticizers for concrete soften the mix before it hardens, increasing its workability, and are usually not intended to affect the properties of the final product after it hardens.

[edit] Plasticizers for plastics

Plasticizers for plastics are additives, most commonly phthalates, that give hard plastics like PVC the desired flexibility and durability. They are often based on esters of polycarboxylic acids with linear or branched aliphatic alcohols of moderate chain length. Plasticizers work by embedding themselves between the chains of polymers, spacing them apart (increasing of the "free volume"), and thus significantly lowering the glass transition temperature for the plastic and making it softer. Some plasticizers evaporate and tend to concentrate in an enclosed space; the "new car smell" is caused mostly by plasticizers evaporating from the car interior.

  • Phthalate-based plasticizers are used in situations where good resistance to water and oils is required. Some common phthalate plasticizers are:
    • Bis(2-ethylhexyl) phthalate (DEHP), used in construction materials, food packaging, children toys, medical devices, and cling wrap
    • Diisononyl phthalate (DINP), found in garden hoses, shoes, toys, and building materials
    • Bis(n-butyl)phthalate (DnBP, DBP), used for cellulose plastics, food wraps, adhesives, perfumes and also in cosmetics - about a third of nail polishes, glosses, enamels and hardeners contain it, together with some shampoos, sunscreens, skin emollients, and insect repellents
    • Butyl benzyl phthalate (BBzP) is found in vinyl tiles, traffic cones, food conveyor belts, artificial leather and plastic foams
    • Diisodecyl phthalate (DIDP), used for insulation of wires and cables, car undercoating, shoes, carpets, pool liners
    • Di-n-octyl phthalate (DOP or DnOP), used in flooring materials, carpets, notebook covers, and high explosives, such as Semtex. Together with DEHP it was the most common plasticizers, but now is suspected of causing cancer
    • Diethyl phthalate (DEP)
    • Diisobutyl phthalate (DIBP)
    • Di-n-hexyl phthalate, used in flooring materials, tool handles and automobile parts
  • Trimellitates are used in automobile interiors and other applications where resistance to high temperature is required. They have extremely low volatility.
    • Trimethyl trimellitate (TMTM)
    • Tri-(2-ethylhexyl) trimellitate (TEHTM-MG)
    • Tri-(n-octyl,n-decyl) trimellitate (ATM)
    • Tri-(heptyl,nonyl) trimellitate (LTM)
    • n-octyl trimellitate (OTM)
  • Maleates
    • Dibutyl maleate (DBM)
    • Diisobutyl maleate (DIBM)
  • Sulfonamides
    • N-ethyl toluene sulfonamide (o/p ETSA), ortho and para isomers
    • N-(2-hydroxypropyl) benzene sulfonamide (HP BSA)
    • N-(n-butyl) benzene sulfonamide (BBSA-NBBS)
  • Glycols/polyethers
    • Triethylene glycol dihexanoate (3G6, 3GH)
    • Tetraethylene glycol diheptanoate (4G7)
  • Polymeric plasticizers.

Some other chemicals working as plasticizers are nitrobenzene, carbon disulfide and β-naphthyl salicylate.

Some plasticizers, eg. DEHP and DOA, were found to be carcinogens and endocrine disruptors.

Safer plasticizers with better biodegradability and less biochemical effects are being developed. Some such plasticizers are:

  • Acetylated monoglycerides; these can be used as food additives
  • Alkyl citrates, used in food packagings, medical products, cosmetics and children toys
    • Triethyl citrate (TEC)
    • Acetyl triethyl citrate (ATEC), higher boiling point and lower volatility than TEC
    • Tributyl citrate (TBC)
    • Acetyl tributyl citrate (ATBC), compatible with PVC and vinyl chloride copolymers
    • Trioctyl citrate (TOC), also used for gums and controlled release medicines
    • Acetyl trioctyl citrate (ATOC), also used for printing ink
    • Trihexyl citrate (THC), compatible with PVC, also used for controlled release medicines
    • Acetyl trihexyl citrate (ATHC), compatible with PVC
    • Butyryl trihexyl citrate (BTHC, trihexyl o-butyryl citrate), compatible with PVC
    • Trimethyl citrate (TMC), compatible with PVC

[edit] Plasticizers for concrete production

Superplasticizers are chemical admixtures that can be added to concrete mixtures to improve workability. Strength of concrete is inversely proportional to the amount of water added or water-cement (w/c) ratio. In order to produce stronger concrete, less water is added, which makes the concrete mixture very unworkable and difficult to mix, necessitating the use of plasticizers and superplasticizers.

Superplasticizers are also often used when pozzolanic ash is added to concrete to improve strength. This method of mix proportioning is especially popular when producing high strength concrete and fiber reinforced concrete.

Adding 2% superplasticizer per unit weight of cement is usually sufficient. However, note that most commercially available superplasticizers come dissolved in water, so the extra water added has to be accounted for in mix proportioning. Adding an excessive amount of superplasticizer will result in excessive segregation of concrete and is not advisable. Some studies also show that too much superplasticizer will result in a retarding effect.

Plasticizers are commonly manufactured from lignosulfonates, a by-product from the paper industry. Superplasticizers have generally been manufactured from sulfonated naphthalene formaldehyde or sulfonated melamine formaldehye, although new generation products based on polycarboxylic ethers are now available. Traditional lignosulfonate based plasticisers and naphthalene and melamine based superplasticisers disperse the flocculated cement particles through a mechanism of electrostatic repulsion (see colloid). In normal plasticisers, the active substances are adsorbed on to the cement particles, giving them a negative charge, which leads to repulsion between particles. Naphthalene and melamine superplasticisers are organic polymers. The long molecules wrap themselves around the cement particles, giving them a highly negative charge so that they repel each other.

Polycarboxylate Ethers (PCE), the new generation of superplasticisers are not only chemically different to the older sulphonated melamine and naphthalene based products but their action mechanism is also different, giving cement dispersion by steric stabilisation, instead of electrostatic repulsion. This form of dispersion is more powerful in its effect and gives improved workability retention to the cementitious mix. Furthermore, the chemical structure of PCE allows for a greater degree of chemical modification than the older generation products, offering a range of performance that can be tailored to meet specific needs.

In ancient times, the Romans used blood as a superplasticizer for their concrete mixes.

[edit] External links

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