Styrenic blends

Styrenic resins, a family of commercially significant polymers and copolymers derived from styrene, rank among the major volume thermoplastic materials used, with an annual consumption of nearly 4Mton/y in the USA alone [Greek, 1991]. Their low cost, ease of processability and good balance of properties account for widespread use. Commercial styrenic resins may be classified into the following types  

Styrene-acrylonitrile copolymer (SAN) and its impact modified versions, viz., ABS (polybutadiene rubber grafted SAN), ASA (acrylate rubber grafted SAN), AES (EPDM rubber grafted SAN) 

Styrene-maleic anhydride copolymer (SMA) and terpolymers with methyl methacrylate (SMA-MMA) and acrylonitrile (SMA-AN) 

Styrene-butadiene block copolymers [di, tri and radial block (S-B) ] 

Among these, polystyrene is the lowest cost, commodity type resin followed by SAN, SMA and the other copolymers. All the styrenic resins are essentially amorphous polymers with glass transition temperatures ranging from about 100 to 130°C, and heat distortion temperatures ranging from about 80 to 120°C, depending upon the comonomer and impact modifier content. 

Styrenic blends find application in more demanding applications. These blends are of considerably higher cost than HIPS or ABS, but offer greater toughness and the option of using a nonhalogen flame retardant (see Section 7). 

Polystyrene and PPO are miscible polymers, and HIPS is typically used in these blends for greater toughness. HIPS-PPO blends can be flame retarded with brominated flame retardants, but perhaps of more interest are blends flame retarded with aromatic phosphates. Phosphates are thought to act through promotion of charring of PPO, but may also have some vapor-phase activity. Typical levels of PPO used to achieve a UL 94 V-0 rating are 30-50% these blends also contain 10-20% of an aromatic phosphate [28]. 

Polycarbonate and ABS are not miscible, but blends are compatible and have excellent toughness. PC-ABS blends are also flame retarded with aromatic phosphates. These blends are typically very high in polycarbonate (70-80%) and also contain at least 10% of an organic phosphate. PC-ABS blends are preferred for unpainted applications because of their excellent UV stability. 

---

Terpolymers from dimethy]-a.-methy]styrene (3,4-isomer preferred)—a-methylstyrene—styrene blends in a 1 1 1 weight ratio have been shown to be useful in adhesive appHcations. The use of ring-alkylated styrenes aids in the solubiHty of the polymer in less polar solvents and polymeric systems (75). Monomer concentrations of no greater than 20% and temperatures of less than —20° C are necessary to achieve the desired properties. 

Other common radical-initiated polymer processes include curing of resins, eg, unsaturated polyester—styrene blends curing of mbber grafting of vinyl monomers onto polymer backbones and telomerizations. 

Organic peroxides are used in the polymer industry as thermal sources of free radicals. They are used primarily to initiate the polymerisation and copolymerisation of vinyl and diene monomers, eg, ethylene, vinyl chloride, styrene, acryUc acid and esters, methacrylic acid and esters, vinyl acetate, acrylonitrile, and butadiene (see Initiators). They ate also used to cute or cross-link resins, eg, unsaturated polyester—styrene blends, thermoplastics such as polyethylene, elastomers such as ethylene—propylene copolymers and terpolymers and ethylene—vinyl acetate copolymer, and mbbets such as siUcone mbbet and styrene-butadiene mbbet. 

Bui et al. (3) prepared glossy cosmetics consisting of poly(styrene-h-ethylene) and poly(styrene-/ -ethylene)-/ -(butylene-/ -styrene) blended with propyl silses-quioxane wax. 

S.C. Tjong and Y.Z. Meng, Effect of reactive compatibilizers on the mechanical properties of polycarbonate/poly(acrylonitrile-buta-diene-styrene) blends, Enr. Polym. J., 36(1) 123-129, January 2000. 

K.H. Pawlowski, B. Schartel, M.A. Fichera, and C. Jager, Flame retar-dancy mechanisms of bisphenol A bis(diphenyl phosphate) in combination with zinc borate in bisphenol a polycarbonate/acrylonitri-le-butadiene-styrene blends, Thermochim. Acta, In Press, Accepted Manuscript, 2009. 

S.K. Gaggar and K. Hongladarom, Weatherable styrenic blends with improved translucency, US Patent 6 720 386, assigned to General Electric Company (Pittsfield, MA), April 13,2004. 

Influence of compatibilization on the foam morphology - despite the significant influence and relevance of compatibilization on the blend structure and interfacial behavior, only partly discussed for polyolefins/styrenics blends [37]... 

K.H. Pawlowski and B. Schartel, Flame retardancy mechanisms of aryl phosphates incombination with boehmite in bisphenol A polycarbonate/acrylonitrile butadiene styrene blends, Polym. Degrad. Stabil., 2008, 93 657-667. 

Pawlowski KH, Schartel B. Flame retardancy mechanisms of triphenyl phosphate, resorcinol bis(diphenyl phosphate) and bisphenol a bisfdiphenyl phosphate) in polycarbonate/acrylonitrile-butadiene-styrene blends. Polym. Int. 2007 56 1404-1414. 

Figure 8. Small-angle x-ray scattering (SAXS) data for poly ( sty rene-b-butadiene-b-styrene) blended with 20% polystyrene and cast from THF/methyl ethyl ketone (after Ref. 64)...

Dion RP and Billovits GF (1996) Interfacial tension a quantitative measure of styrenic blend compatibility. Polym Prepr (Am Chem Soc, Div Polym Chem) 32 529-30. 

To improve the notched impact strength and to prevent hydrolysis induced embrittlement of PBT, PBT-styrenic blends have been thoroughly investigated. In comparison with ASA-PC, blends of ASA and PBT generally have to be compa-tibilized in order to yield acceptable mechanical properties. In the case of glass fibre reinforced PBT-ASA blends, an added and important bonus is reduced warpage properties on injection moulding in comparison with reinforced PBT. 

Flame-retardant polystyrene is used primarily in expanded foam for building insulation. Rubber-modified styrenic polymers are flame retarded for use in a number of applications, such as enclosures for electronics and business equipment. By far the largest volume flame-retardant HIPS application is television enclosures (Figure 29.1) these are made primarily from flame-retardant HIPS [3]. Flame-retardant HIPS has an attractive balance of mechanical properties, processability and cost. Flame-retardant styrenic blends such as HIPS-PPO and PC-ABS also find utility in a number of electrical applications such as printers, computers and monitors. These blends have received increasing attention recently because of their ability to be flame retarded with nonhalogen flame retardants (see Section 7). 

Flame-retardant styrenic polymers find utility in applications such as building insulation (expanded polystyrene foam) and electronic enclosures (flame-retardant HIPS, ABS and styrenic blends). The most effective flame retardants are halogen-(particularly bromine)-containing compounds these flame retardants act by inhibiting the radical combustion reactions occurring in the vapor phase. Flame-retardant plastics are in a state of flux, due to influences of... 

Constant-time imaging of H in solids has been applied in combination with MAS for line narrowing [Cor3, Cor4]. The images were recorded for a dynamically stressed poly(isoprene) phantom and of poly(butadiene) in two poly(butadiene)/poly(styrene) blends. Spectroscopic MAS imaging has also been tested on deuterated polymers to probe differences in molecular moblity from the lineshape of the rotary-echo envelope... 

Fig. 8.5.3 Proton MAS images of solid discs (750 iJtm thick and 3.5 mm diameter) of poly(butadiene)-poly(styrene) blends, (a) Mechanical blend of both components, (b) Blend cast from solution in toluene. The image contrast is caused by the differences in the strengths of the dipole-dipole couplings among H for the two polymers. The signals from poly(styrene) are filtered out (white) so only poly(butadiene) protons contribute to the image (dark). The spatial resolution is better than 50 p-m at a spinning frequency of 5 kHz. Adapted from [Cor4]. Copyright 1989 American Chemical Society.

The second large group of styrenic blends comprises these with polyolefins — they are summarized in Tables 1.11 and 1.12. These blends are mainly used in packaging. Formulated for extrusion, injection and blow molding, they show excellent processability, improved impact strength, low moisture absorption and shrinkage. 

In PO blends the preferred compatibilizer has been EPR, while in styrenic blends SBS or SEES maintain high visibility in spite of the price. Both compatibilizers can also improve the impact strength. However, excessive amount of elastomer can lower the modulus and strength of the alloy, thus the elastomeric particles size should be optimized. 

Styrenics blends used for blow molding include SAN/elastomer Luran S) and ABS/BA or PB (Terluran ). The high viscosity grades are the most suitable. Parisons should be extruded at 220-230°C. In some cases, the temperature might be kept as low as 200-210°C to avoid sagging. 

Polystyrene also forms miscible blends with PPE in which the hydrogen-transfer reaction of PS is rendered less effective due to the PPE component [Jachowicz et al., 1984]. In contrast, poly(a-methyl styrene) blends with PPE, although also reported miscible, show no evidence of any interactions during degradation. Head-to-head and head-to-tail PS structures were found to behave (hfferentiy [JCryszewski, et al., 1982]. 

Styrenic blends ABS blends ABS/PC Monsanto Triax 2122 Partial miscibility Amorphous/ Amorphous... 

PVC has been blended with some styrenic resins primarily to achieve some degree of flame retardant characteristics and cost benefits. ABS, SMA, and rubber modified SMA, SMA-MMA copolymers have been used commercially for blending with PVC. These have been discussed under the styrenic blends and illustrated in Table 15.4. 

Compatibilisers (adhesion promoters) Improving the interaction between different phases SBS in styrenic blends, epoxidised or maleated functionalised polymers in general... 

There are a number of polymeric compatibilisers, e.g., block/graft copolymers like tri-block copolymers of SBS used mostly in styrenic blend compatibilisation, and functionalised polymers with certain functional groups (epoxidised or maleated), which can act like a surfactant . 

---