Guardian (polymer)

Polypropylene is processed to create pellets.[2] These pellets are extruded from a core extruder in conjunction with polyolefin pellets from two "skin layer" extruders, and are combined into a molten polymer.[2][3][4] This consists of a 37.5µm thick polypropylene sheet sandwiched between two 0.1 µm polyolefin sheets,[4][3] creating a thin film 37.7 µm thick.

The molten polymer undergoes snap cooling as it passes by gravity feeding through a brass mandrel, which imparts on the thin film many properties, including its transparency.[2] The cast tube material is then reheated and blown into a large bubble using air pressure and temperature.[2] At the base of the four-storey chamber convergence rollers collapse the tube into a flat sheet consisting of two layers of the thin film.[4][2] This creates the base biaxially-oriented polypropylene substrate of 75.4 µm thickness, called ClarityC by Innovia Films.[3][5]

The base substrate is slit as it exits the convergence rollers.[2][4] Four 3-micrometre (0.00012 in) thick layers of (usually white) opacifier are applied to the substrate, two on the upper surface and two on the lower surface.[3][4] A mask prevents the deposition of the opacifier on parts of the substrate that are intended to remain transparent.[6] These overcoat layers protect the substrate from soiling and impart on it its characteristic texture,[7] and increase the overall thickness to 87.5 µm. The resulting product is the Guardian substrate.[4]

The opacifier conversion phase involves the use of resin and solvents, creating volatile organic compounds (VOCs) as by-products that are combusted in a thermal oxidizer.[5] The resulting polymer substrate then passes through a rotary printing press using chrome-plated copper cylinders.[5] After printing, the holographic security foil is incorporated into the base substrate.[5] This is then cut into sheets and transported to the banknote printing companies in wooden boxes as a secure shipment.[5][8]

Guardian is a non-porous and non-fibrous substrate.[2] Because of this, it is "impervious to water and other liquids", and so remains clean for longer than a paper substrate.[2] It is difficult to initiate a tear on the substrate, which has higher tear initiation resistance than paper.[2]

Guardian is used in the printing of polymer banknotes by many central banks.

It is the base material used for currencies printed by:

In 1993, the Bank of Indonesia issued a commemorative Rp 50,000 banknote and the Central Bank of Kuwait issued a د.ك1 banknote.[4] In 1998, the Bank Negara Malaysia issued a commemorative RM 50 banknote,[4] and the Central Bank of Sri Lanka issued a commemorative Rs200 banknote.[27] In 1999, the Northern Bank of Northern Ireland issued a commemorative £5 banknote,[28] and the Central Bank of the Republic of China in Taiwan issued a commemorative NT$50 banknote.[29][4] In 2000, the Central Bank of Brazil issued a commemorative R$10 banknote[30] and the People's Bank of China issued a commemorative ¥100 banknote.[4] In 2001, the Central Bank of Solomon Islands issued a commemorative SI$2 banknote.[31] In 2009, the Bank of Mexico issued a commemorative $100 banknote.[4]

• Guarner, Enrique (April 2009). "Polymer and the printing process" (PDF). Billetaria. Cash and Issue Department, Bank of Spain. 3 (5): 26–27. Retrieved 9 March 2014.