Sealing device and back-up ring

A sealing device comprising:

• a rod packing configured to seal an annular clearance between a housing and a shaft of a hydraulic cylinder used in construction machinery;
• and
• a buffer ring for countering a hydraulic pressure that acts on the rod packing,
• wherein:
• the buffer ring includes
• an annular packing formed from an elastic material and disposed within the clearance, and
• a back-up ring in contact with the packing,
• the back-up ring has a contact surface in contact with the shaft,
• a body of the back-up ring including the contact surface comprises a resin composition that includes a base material and a filler material,
• the base material comprises a polyamide resin,
• the filler material comprises a low friction resin compatible with the polyamide resin such that the filler material is dissolved in the base material to create a homogenous mixture of the base material and the filler material throughout the entire body of the back-up ring, and
• the filler material has a lower static friction coefficient with respect to the shaft than the polyamide resin.