Ether
Thermoplastics > Polyethers
Ether group | ||||||||
One of the most important bonding group is the ether group (-O-). Acetal polymers, also known as polyoxymethylene (POM) or polyacetal, are formaldehyde-based thermoplastics. An acetal is a functional group with the following connectivity R-H-C- (O-R ') - (O-R "), where the three R groups are organic fragments. The central carbon atom has four bonds and, therefore, is saturated and has a tetrahedral geometry. The R 'and R "groups may be equivalent to each other (a" symmetric acetal ") or not (a" mixed acetal "), and one or both may be hydrogen atoms instead of organic fragments. The acetals are formed and converted into carbonyl compounds (aldehydes or ketones R2C = O). The term ketal is used to identify structures with ketones in place of aldehydes and, historically, the term acetal was used specifically for aldehyde cases. The formation of an acetal occurs when the hydroxy group of a hemiacetal is protonated and lost as water. The carbocation that is produced is attacked quickly by a molecule of alcohol. The proton of the alcohol attached gives the acetal. |  | |||||||
| Acetal polymers | ||||||||
Acetal polymers, also known as polyoxymethylene (POM) or polyacetal, are formaldehyde-based thermoplastics. Poly-formaldehyde is thermally unstable. It decomposes on heating to produce formaldehyde gas. In time two methods were developed to stabilize the polyformaldehyde. A method for making polyacetal produces a homopolymer through the condensation reaction of polyformaldehyde and acetic acid (or acetic anhydride). Acetic acid puts acetate groups (CH3COO-), which provide thermal protection against decomposition to formaldehyde, at the ends of the polymer. The second method for the production of polyacetal produces a more stable copolymer product by the reaction of trioxane, a cyclic trimer of formaldehyde and a cyclic ether, such as ethylene oxide. The improved thermal and chemical stability of the copolymer against the homopolymer is the result of randomly distributed oxyethylene groups. The copolymer is also more stable than the homopolymer in an alkaline environment. Its oxyethylene end cap is stable in the presence of strong bases. Further stabilization of the acetal polymers also includes the addition of antioxidants and acid scavengers. Polyacetals are subject to oxidative and acid degradation, which leads to a decrease in molecular weight. Once the homopolymer chain is broken by such attacks, the exposed poly-formaldehyde ends can be decomposed into formaldehyde and acetic acid. |  | |||||||
| Difference between homopolymer and copolymer | ||||||||
The homopolymer is more crystalline than the copolymer. The homopolymer provides better mechanical properties, with the exception of elongation. The oxyethylene groups of the copolymer provide improved long-term chemical and environmental stability. The chemical stability of the copolymer results in a better retention of the mechanical properties during a prolonged shelf life of the product. Polyphenylene ether plastics are also known as polyphenylene oxide (PPO). PPE materials are mixed or alloyed with other plastics, which is why they are called modified PPE or PPO. PPE is compatible with polystyrene (PS). |  | |||||||
Phenolic group | ||||||||
Polymer in which the repeating structural unit is the oxyphenylene radical. It is a high temperature thermoplastic of general formula (C8H8O) n. Pure PPE is very difficult to transform, so mixtures are made to improve its injection capacity. It is mainly used as a mixture with polystyrene, with high impact styrene-butadiene copolymers or with polyamide. These polymeric mixtures of PPO (phenylene polyoxide) with other polymers, make up a family with a wide combination of mechanical, thermal and electrical properties. In a general way, they have the lowest moisture absorption within engineering plastics, optimum dimensional stability, exceptional resistance to hydrolysis, low density and high thermal distortion temperature, being able to work up to 143ºC, also at room temperature and at high temperatures. the retention of properties and dimensional stability in the presence of water and in high humidity and vapor environments. |  | |||||||
| Blends | ||||||||
PPO / Polystyrene This mixture composes a family with a wide combination of mechanical, thermal and electrical properties PPO / Polyamide This class was specially developed to offer high temperature resistance, good chemical resistance and dimensional stability. PPO / Polypropylene It has superior properties to most polyolefins, low moisture absorption, excellent chemical resistance, good hydrolytic stability. |  | |||||||
| Polymers of the ether group | ||||||||
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