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The most common types of specialty plasticisers are:
Aliphatic dibasic acid esters
Cyclohexane diacids esters
Glycerol Acetylated esters
These types of plasticisers are based on aliphatic dibasic acids with carbon numbers ranging from C5 (glutaric) to C10 (sebacic).
Adipates are by far the most important esters in this group. Alcohols of similar chain length to those used in phthalate manufacturing can be esterified with adipic acid, rather than phthalic anhydride, to produce a range of adipate plasticisers. For example, esterification of 2-ethylhexanol with adipic acid yields Di-2-ethylhexyl adipate (DEHA), also known as di-octyl adipate (DOA).In PVC applications, adipates offer enhanced low temperature properties compared to phthalates. In plastisol applications, adipates impart low plastisol viscosities due to their lower neat viscosities.
Adipates are typically based on alcohols in the C8 to C10 range. Incompatibility problems can be encountered at higher carbon numbers, especially at high addition levels. Relative to phthalates, adipates are more volatile, exhibit poorer fusion and compatibility with PVC, have higher migration rates, and are generally more costly. As a result, it is not uncommon to use adipates in blends with high phthalates to attain the desired combination of properties.
Di-2-ethylhexyl sebacate (DOS) and di-2-ethylhexyl azelate (DOZ) are the most common members of this group, but di-isodecyl Sebacate (DIDS) is also used.
These plasticisers impart low temperature performances superior to adipates but also command a significant premium, and their use is generally limited to extremely demanding low temperature flexibility specifications (e.g. underground cable sheathing in arctic environments).
Di-benzoate plasticisers are obtained by direct esterification of benzoic acid with glycols for use primarily in non PVC applications such as PVAc based adhesives, latex caulks and polysulfide sealants.
The most important benzoate plasticizer commercially, di-propylene glycol dibenzoate, a high solvating plasticiser, can also been proposed as a one-to-one replacement of the low phthalate BBP. Tailor-made blends (di- and triethylene glycol and di-propylene glycol dibenzoates) have also been offered as stain resistant plasticisers for flooring. Plastisols prepared with di-benzoates, that tend to solidify below 16°C, exhibit higher initial viscosities and very poor storage stability.
Spread coating applications, like flooring or wall-covering, require plasticisers with low neat viscosity, fast fusing properties for higher processing speed. In recent years, production of monobenzoates which include isodecyl benzoate (C10 alchol) and isononyl benzoate (C9 alcohol) have received increased interest.
Citric acid is the starting material for a number of citrate ester plasticisers, such as tributyl citrate, acetyl tributyl citrate, triethyl citrate, acetyl triethyl citrate and tri-2-ethylhexyl citrate. These plasticisers are used in a variety of primarily vinyl materials such as vinyl resins and films, but also in some cellulose
acetates. They are used to plasticise vinyl resins employed in applications such as toys, pacifiers , medical devices and packaging films.
Of the approximately 230,000 tons of citric acid used annually in Western Europe, 58% are used in food and beverage applications, 24% in household detergents and cleaners, 9% in pharmaceuticals and 9% in industrial applications.
Citrates lack the permanency offered by the high phthalates, for a much higher cost. They exhibit higher volatility loss, higher fogging, higher level of extraction, limiting their use for the production of durable goods such as cables, flooring or roofing membranes.
Tributyl citrate is used in PVC, polyvinyl chloride/vinylidene chloride copolymers or polyvinyl chloride/vinyl acetate resins that are subsequently used for items such as food-wrapping film. Acetyl Tributyl citrate presents several advantages because of its higher heat stability and does not discolour when processed in compounded resins compared to non acetylated citrates.
Expoxy plasticizers are esters containing an epoxy group such as epoxidized soybean oil (ESBO) and epoxidised linseed oil (ELO). They are formed by the oxidation of an olefinic double bond to an oxirane structure. Due to the presence of the epoxy group, these plasticisers are used to improve heat stability in the production of PVC articles by techniques such as extrusion, calendering, injection moulding, rotational moulding and spread coating. They are also used in rubbers, epoxy resins, paints and coatings. As co-stabilisers, they are used in the range of 1-5 phr (parts per hundreds of PVC), but they can also act as lubricants. The most common problems associated with the use of epoxy plasticizers at higher concentrations are the development of incompatibility with PVC during aging, leading to migration and development of tacky surfaces in the presence of sunlight.
The principal advantage of phosphate esters is their improved fire retardancy compared to phthalates. The fire performance of PVC, relative to other polymeric materials, is extremely good due to its high halogen content, but the addition of certain plasticisers may impair this property. Consequently, there is a need, especially in demanding applications, to improve the fire retardant behaviour of flexible PVC.
Triaryl phosphates and alkyl diaryl phosphates are the two important categories of flame retardant phosphate plasticisers. Phosphate esters can help produce low smoke, low flammable flexible PVC.
Tris (2-ethylhexyl) phosphate shows good compatibility with PVC and imparts good low temperature performance in addition to good fire retardancy.
2-ethyhexyl diphenyl phosphate is widely used in flexible PVC applications due to its combined properties of plasticising efficiency, low temperature performance, good UV stability and migration resistance.
Polymeric plasticizers are polyesters produced from polyhydric alcohols (diols) that have been esterified with dibasic acids, commonly adipic acid, in the presence of mobobasic acids or alcohols. Those plasticiser are characterized by a very high molecular weight offering high resistance to migration as well as resistance to extraction by fats, oils and hydrocarbons. They offer better resistance to extreme temperatures but provide little benefits in processability.
They can be used as sole plasticisers when the greatest permanence is required but their high viscosity, and poor solvating properties make them very difficult to process, particularly in plastisols.Their typical use is in flexible films in contact with fatty food or to resist migration into adhesives, coatings or any other plastic materials in close contact with flexible PVC.
Secondary plasticisers, also known as extenders, continue to play a role in flexible PVC formulations. They do not impart flexibility to the PVC resin on their own but, when combined with a primary plasticiser, they will add flexibility to the final product. The majority of secondary plasticisers in use are chlorinated paraffins, which are hydrocarbons chlorinated to a level of 30-70%, typically 52%.
For a given hydrocarbon chain, viscosity increases with chlorine content, as does the fire retardancy imparted to the formulation (these materials aid fire retardant due to their chlorine content). Chlorinated paraffins of the same chlorine content may, however, have different volatilities and viscosities if they are based on different hydrocarbon chains. As well as imparting improved fire retardancy these materials may also result in volume cost savings.
Precise knowledge of the compatibility between standard plasticisers and chlorinated paraffins is required since, at certain temperatures, some mixtures become incompatible with each other and the PVC resins in use. Phthalate-chlorinated paraffin compatibility decreases as the molecular mass of the phthalate and the plasticiser content of the PVC formulation increase.
Other materials which are often referred to as secondary plasticisers include epoxidised soybean oil (ESBO) and epoxidised linseed oil (ELO). These can act as lubricants but also act as secondary stabilisers for PVC due to their epoxy content which can remove HCl from the degrading polymer.
Di-isononyl cyclohexane dicarboxylate has been developed for use in sensitive applications where exposure issues to low phthalates were of great concern such as in medical devices or toys. This plasticiser can be produced by the selective hydrogenation of the aromatic ring in di-isononyl phthalate (DINP), in the presence of a noble catalyst.
Such plasticisers can offer improved low temperature performance, reduced plastisol viscosities and improved stability to UV-light exposure.
Cyclohexanoates are more volatile and less compatible with PVC, compared to a phthalate with similar molecular weight and alcohol carbon number distribution. They also require higher fusion and processing temperature.
Di-isononyl cyclohexane dicarboxylate is used essentially in sensitive applications like medical, toys and food contact.
Terephthalates are the other commercial isomeric form of phthalates. Terephthalates are esters of tere-phthalic acid. Terephtalates plasticisers include the 1,4 benzenedicarboxilic acid ester sometimes referred to as DEHTP (di-(2ethylhexyl) terephthalate) or DOTP di-octyl terephthalate. DEHTP compared to DEHP offers better low temperature properties, better resistance to soapy water extraction and lower volatility. In plastisols, it provides lower initial viscosity and better viscosity stability but requires higher fusion and processing temperature. Terephthalates exhibit lower levels of compatibility with PVC than ortho-phthalates. DEHTP is lacking the permanence properties offered by the high phthalates, limiting its use for the production of durable flexible PVC articles.
Different types of glycerol esters have been proposed as alternatives to low phthalates, their limited availability and higher costs currently limit their use.
Trimellitates are produced by the esterification of C7-C10 alcohols with trimellitic anhydride (TMA), which is similar in structure to phthalic anhydride with the exception of a third functionality on the aromatic ring. Consequently, esters are produced in the ratio of three moles of alcohol to one mole of anhydride. Common esters in this family are Tris-2-ethyhexyl trimellitate (Tri-octyl trimellitate - TOTM), L79TM, an ester of mixed semi-linear C7 and C9 alcohols, and L810TM, an ester of mixed C8 and C10 linear alcohols.Due to their low volatility, these plasticisers are used in the automotive industry (dashboard PVC skin produced by slush moulding) and in the insulation or sheathing of electrical cables. The lower migration rate and extraction resistance of these plasticisers are their main advantage. . The low volatile loss makes them suitable for use in automotive interior applications where the windscreen fogging performance is important. Their higher viscosity and poorer fusion compared to high phthalates make them more difficult to dry-blend (compounding) and process.
This plasticiser is made from fully hardened castor oil and acetic acid. Castor oil is extracted from the seeds of the castor oil plant, which is an annual plant grown in India, Brazil and China. The castor oil contains between 85% to 95% ricinoleic acid. The performance of castor oil is improved by modifying its structure (hardening) and replacing the longer chain acids with acetic acid. The resulting fully acetylated glycerol monoester has a lower molecular weight, improving the compatibility and processability of the plasticiser.
Expected main PVC applications for such esters are toys, bottle cap liners, screw cap liners for e.g. jam, teething rings, cling film, tubes and conveyor belts in the food industry and medical equipment. They also find a limited use in calandered homogene vinyl flooring, stimulated by “green” marketing.
Harvesting time and seasonality linked to the production of hardened castor oil currently limits the production capacity and availability of such plasticisers. They are produced at much higher costs than high phthalates.