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Lec-5: Engineering Resins & Their Blends………………Eng. Auda Jabbar M.Sc.

Lec-5Engineering Resins and Their Blends

The term engineering polymer is applied to a processable resin, capable of being formed to precise and stable dimensions, having high performance at CUT≥ 100°C, and the tensile strength of σ ≥ 40 MPa . Five polymer families belong to this category: PA, PEST, PC, POM,and PPE.Engineering resins and their blends have been foamed:1- Using mainly chemical foaming agents, e.g.,hydrazodicarboxylates, benzazimides or 5-phenyltetrazole. However, products of decomposition of these agents (e.g., alcohol, ammonia, water, etc.) were found to hydrolyze PC, PA, PC/ABS, or PEST, reducing the performance of foamed products. 2- For this reason, dihydrooxadiazinones were proposed as the preferred chemical foaming agents for the engineering resins, their compositions and blends.

Dihydrooxadiazinones

3-Another method for foaming high temperature polymers profits from thermal instability of polypropylene carbonate (PPCO, MW = 1 to 1000 kg/mol).


The process involves blending PPCO with a resin or its blend that is to be foamed. The blend is then heated to T > 300°C, sufficient to melt the principal resin and to decompose PPCO. Talc, wollastonite, clay, CaCO3 or citric acid can be used as a nucleating agent. The densities of the molded articles were reduced by at least 50%.4- Most recently, foaming of the engineering resins by supercritical CO 2

was introduced.1- Polyamides (PA):

Polyamides are abbreviated as PA-, followed by the number of carbons in a di-amine and dicarboxylic acid, viz. PA-66 has six carbons in each component, etc.- Poly-ε-caprolactam or polyamide-6 (PA-6) was first studied by Von Brown in 1905-10, and 30 years later commercialized as Perlon™. - In the year 1936, PA-66 and several other aliphatic and semi-aromatic polyamides were disclosed- Two years later, DuPont introduced Nylon™, an aggregate of New York and LONdon.

- PA-11 was commercialized in 1955,- PA-12 in 1966,- PA-612 in 1970,- PA-46 in 1987. - In 1976 DuPont started production of the super tough PA-blends .

Super tough PA-blends - Liquid crystal aromatic polyamides (PARA), poly(meta-phenylene isophthalamide), Nomex™, and poly(para-phenylene terephthalamide), Kevlar™-49, were commercialized in 1961 and 1965, respectively.


- Amorphous aromatic polyamide, Trogamid™, was introduced in 1969, and polyphthalamide Amodel™ in 1991.

Table : Comparative properties of PA’s

PA started to be blended with other engineering resins at the end of the 1960’s, viz. PA with POM ; PA with POM and PET; PA-6 reactivelycompatibilized with PET ; PA-6-co-diisocyanate copolymer with PET. By the year 1970 the number of PA blends rapidly started to increase. The main efforts were directed toward improvement of toughness and processability of PA. Reactive compatibilization and impact modification became an integral part of the PA-blends’ technology.PA(1)/PA(2) Blends: These blends are commercially available, e.g., Zytel 3100™, Grilon™ BT. They show improved processability, solvent resistance, elongation, low temperature impact and tensile strength, as well as enhanced barrier properties (see Table ):


PA/PPE(Polyphenyle Ether) Blends: PA’s are excellent candidates for blending with PPE — each ingredient compensates for deficiency of the other. Since the resins are immiscibleand brittle, they must be compatibilized and toughened. In consequence, PA/PPE blends comprise minimum four polymeric components: PA, PPE, a styrenic modifier and an acidic compatibilizer. Usually PA is the matrix in which PPE/styrenic resin domains are dispersed.

PA/Specialty Polymer Blends: Addition of a small amount of PA improves processability of the specialty resin and is beneficial to performance of the GF-reinforced systems. Addition of specialty resin to PA enhances the thermal behavior and rigidity of the latter resin. For higher concentration of these ingredients, compatibilization and impact modification are required. Since PA is sensitive to heat, oxygenand moisture, the compounding requires a special care.

2-Thermoplastic Polyesters (PEST):

- PEST Blends with PC: The PEST/PC blends are immiscible (Tg of PC is depressed by ca. 20°C) and brittle, requiring toughening.The main advantage of the PC/PEST alloys is the increased stiffness, reduced susceptibility to stress cracking on contact with fuels, and an improved resistance to chemicals and fuels.


The blends show good processability, heat resistance, ductility, HDT, high modulus at high temperature, good electrical properties, thermal stability, impact, tensile and flexural strength over a wide temperature range, low shrinkage, good dimensional stability, but they may have poor weatherability, and their solvent resistance(while superior to that of PC) is moderate

- PEST Blends with PPE Blending PPE with either PEST or PC poses similar problems — the polymers are immiscible and brittle, hence require compatibilization andtoughening. The PEST/PPE blends are multicomponent systems, with ≥ 5 components: PEST, PPE, styrenic copolymer, compatibilizer, and impactmodifier. For improved modulus and dimensional stability, they are usually reinforced with GF. These alloys are known for excellent processability, high solvent resistance, and dimensional stability.

-Polyurethanes (TPU):The currently available TPUs can be divided mainly in two groups, based on soft segment chemistry:

1. polyester-based TPUs (mainly derived from adipic acid esters)2. polyether-based TPUs (mainly based on tetrahydrofuran (THF)

ethers).

The polyester-type thermoset polyurethanes were commercialized in 1942, and the linear thermoplastic ones (TPU) ten years later.


Owing to great diversity of the ingredients, the TPU performance can be readily modified. For this reason, as well as because of the cost, TPU’s are seldom blended. Their use can be divided into three groups: 1- Blends with POM(Poly oxymethylene)

POM2- Blends in which TPU is used as a compatibilizer and impact modifier, 3- Others.

Among the features of commercially available TPU are:

Excellent abrasion resistance Outstanding low-temperature performance Excellent mechanical properties, combined with a rubber-like

elasticity High shear strength High elasticity High transparency Good oil and grease resistance

3- Polycarbonate (PC):

Polycarbonates are polyesters of polyhydric phenols and carboxylic acid. Except for the lack of crystallinity, their properties resemble those of PEST. The most common polycarbonate, that of bisphenol-A (PC), was commercialized in 1956. PC is tough, transparent, self-extinguished, dimensionally stable, resistant to salts and oxidation, but susceptible to abrasion, stress cracking and attacks by solvent, acids and alkali. Its Tg = 149°C, but the ductile-brittle transition is at 0-10°C.


The resin ought to be toughened, for example by addition of ABS, MBA or MBS. Originally, the commercial PC resins were linear polymers with high shear viscosity and low melt strength, thus difficult to process in operation involving extensional flows, viz. blow molding stretching, foaming. Several years ago branched PC (bPC) became available. The resin is usually blended with linear PC at the ratio that on the one hand is economically viable and on another that provides sufficient melt strength for the required process.

PC Blends with Specialty Resins: There is a great diversity of polyimides (PI) having Tg = 180-420°C.

Several were blended with PC to improve its stiffness, HDT and strength. PEI/PC blends were commercialized in 1992 as Ultem™ LTX, for injection molding or extrusion.


They show higher impact resistance than PEI and higher heat resistance than PC, as well as they retain the stain, chemical resistance, and thehydrolytic stability of PEI.

4- Polyoxymethylene (POM):

Polyoxymethylene (POM), also known as acetal,[1] polyacetal and polyformaldehyde, is an engineering thermoplastic used in precision parts requiring high stiffness, low friction and excellent dimensional stability.

POM is crystalline, thus rigid, brittle, and chemically non reactive. The most common POM blends are homologous mixtures of POM’s having different molecular structure (linear, branched, crosslinked) , different molecular weight , or with different end-groups. To improve weatherability of POM, the resin was blended with PMMA and a fluoropolymer (viz. PTFE, PVF, PVDF)

http://en.wikipedia.org/wiki/Thermoplastic

http://en.wikipedia.org/wiki/Polyoxymethylene#cite_note-1


5- Polyphenylene ether (PPE): In 1956, by oxidative coupling of 2,6-dimethyl phenol, poly(2,6-dimethylphenyl ether) was obtained (PPE). PPE is amorphous (Tg = 210°C), but it can crystallize (Tm = 257°C). It is thermally stable only to T ≤ 150°C (CUT = 125°C). It has good rigidity, creep resistance, dimensional stability, high electrical, chemical, moisture and flame resistance. The main disadvantages are: processability, oxidative degradation, low impact strength, and weatherability. The resin is usually “plasticized” by blending with styrenics.►Owing to miscibility of PPE with PS, the compatibilization is relatively simple. However, blending PPE with either PA or PEST is more challenging, since these systems require reactive compatibilization►PPE blends with polytrans octanylene (PTO) can be processed at 260°C and have HDT≥ 194°C. The patents suggested that PTO is miscible with PPE

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