Multilayer tubes

A multilayer tube comprising two or more layers including a layer (a) and a layer (b), wherein

• at least one pair of the layer (a) and the layer (b) are disposed adjacent to each other,
• the layer (a) contains an aliphatic polyamide composition (A),
• the layer (b) contains a semi-aromatic polyamide composition (B),
• the aliphatic polyamide composition (A) contains a polyamide (A1), a polyamide (A2) and an elastomer polymer (A3),
• the polyamide (A1) is an aliphatic polyamide having a ratio of the number of methylene groups to the number of amide groups of not less than 8.0, and is not poly(caproamide/dodecanamide) copolymer (polyamide 6/12), and is contained in the aliphatic polyamide composition (A) in an amount of not less than 40% by mass and not more than 90% by mass,
• the polyamide (A2) is a polyamide other than the polyamide (A1), and is not polycaproamide (polyamide 6) and polyhexamethylene sebacamide (polyamide 610), and is contained in the aliphatic polyamide composition (A) in an amount of not less than 5% by mass and not more than 30% by mass,
• the elastomer polymer (A3) comprises structural units derived from an unsaturated compound having a carboxyl group and/or an acid anhydride group, and is contained in the aliphatic polyamide composition (A) in an amount of not less than 5% by mass and not more than 30% by mass,
• the difference in absolute value between the solubility parameter SP of the polyamide (A1) and that of the polyamide (A2) [|(SP of polyamide (A1))—(SP of polyamide (A2))|] is not less than 1.8 (MPa)1/2 and not more than 5.5 (MPa)1/2,
• the semi-aromatic polyamide composition (B) contains a semi-aromatic polyamide (B1) or a semi-aromatic polyamide (B2),
• the semi-aromatic polyamide (B1) or the semi-aromatic polyamide (B2) is contained in the semi-aromatic polyamide composition (B) in an amount of not less than 60% by mass,
• the semi-aromatic polyamide (B1) comprises 50 mol % or more C10 aliphatic diamine units based on all diamine units in the semi-aromatic polyamide (B1), and 50 mol % or more terephthalic acid units and/or naphthalenedicarboxylic acid units based on all dicarboxylic acid units in the semi-aromatic polyamide (B1),
• the semi-aromatic polyamide (B2) comprises 50 mol % or more xylylenediamine units and/or bis(aminomethyl)naphthalene units based on all diamine units in the semi-aromatic polyamide (B2), and 50 mol % or more C9 or C10 aliphatic dicarboxylic acid units based on all dicarboxylic acid units in the semi-aromatic polyamide (B2),
• wherein [A]>[B]+10 wherein [A] is the concentration (μeq/g) of terminal amino groups and [B] is the concentration (μeq/g) of terminal carboxyl groups, per 1 g of each of the polyamide (A1) and the polyamide (A2) in the aliphatic polyamide composition (A), and
• wherein the semi-aromatic polyamide composition (B) contains the elastomer polymer (A3).