Plastics colour change

Open nozzles with direct gate are generally of simple design (Figure 4.14), and the melt is not left behind in them, so that they are best suited to heat sensitive plastics and to plastic colour changes. 

Plastic colour change. The time needed to change the colour or type of plastic depends primarily on the nature of the HR system and on the colour change sequence. 

Radiation sterilization Radiochromic chemical Plastic devices impregnated with radiosensitive chemicals which undergo colour changes at relatively low radiation doses Only indicate exposure to radiation... 

In some applications, particularly in the nuclear industry, medical instrumentation and food sterilisation, plastics are subjected to damage, which is associated with a loss in molecular weight and deterioration of some physical properties, such as impact strength and colour change upon exposed the plastics to gamma rays [65, 66]. 

An electrically controlled flexible colour-changing device can be obtained when an electrochromic material is apphed to textile. The first patent for such a structure was published in 1976 [70]. Since then, more than 30 patents have been pubhshed, with a notable increase since 2010, showing the considerable gain of interest in this subject. Over the same time, the role played by the textile material in these structures has increased. The 1976 patent describes a structure composed of two glass or plastic electrodes, coated with tin dioxide (as the electrode) and tungsten trioxide (as the... 

Most recently Beaupre et al. developed a flexible electrochromic device using textile in 2006 [71]. The structure is made with a transparent electrode, covered with spray-coated electrochromic polymer, a gel electrolyte and finally with a conductive textile. The textile electrode is made with a textile fabric coated with copper and nickel. The other electrode is made of glass or polyester (PET) coated with ITO. Two electrochromic conductive polymers have been tested. Similar colours and colour changes are obtained for structures using two PET-ITO electrodes, or two glass-ITO electrodes, or one textile electrode with one PET-ITO electrode. The colour change is visible but slow. When a plastic electrode and a textile electrode are used, the structure is flexible. A similar structure, using a copper-coated textile cathode, was described by Zhan et al. in 2013 [72]. 

In 2014 Yan et al. developed a flexible, extensible electrochromic device [74]. The electrochromic material (tungsten trioxide) is electrochemically deposited on a flexible, extensible and transparent polydimethylsiloxane (PDMS) matrix coated with silver. The entire structure is extensible (up to 50%) and presents a visible colour change, from white to blue. The colouration occurs in 1 s and the discolouration in 4 s. After 100 cycles, the contrast between the oxidized and the reduced state only decreases by 19%. A small display composed of three independently controlled pixels was realized. The PDMS-based structure has been combined with a cotton fabric in order to form a textile-based electrochromic device. However, the textile itself can hardly be considered as indispensable for the working of this structure. It is more a plastic-based electrochromic structure that can be fixed on textile than a textile electrochromic structure. 

A colour change that is triggered by light rather than heat has been introduced by Polycolour Plastics, with its new Dual Colour system. 

Ultraviolet radiation can cause colour change and degradation of physical properties, especially in polyolefins, ABS, PVC, PC, and PU. Stabilizers to protect plastics compounds against the effect of UV light (especially sunlight) act to prevent oxidation from that source. To counteract this, UV additives offer three... 

An HR system must be selected for a particular application and plastic. A change of colour of the plastic may be difficult and time-consuming if this has not been allowed for in the design (this is also true of systems with internal heating). A change in the type of plastic may be difficult or even impossible if, for example, nozzles of a different type have to be used for the new plastic. 

Tip nozzles are equipped with a full body, a partial body, or - in the case of torpedoes - no body at all. The nozzle body facilitates the use of insulating gaps, and thus assists the processing of crystalline plastics and colour changes. Nozzles designed for injection of reinforced and filled plastics are supplied as a special product. 

A colour change may also cause problems, because the melt flows by a set route, the shortest, and the dead spots are substantially larger than in other designs (Figure 4.30b). When POM plastic in particular is being injected, use of several small but quite separate insulating chambers is recommended (Figure 4.30c). 

Because of the relatively low gate temperature, good conditions were created for the injection of thermally-stable amorphous plastics with an injection temperature of less than 300 °C, and for injection of slow-setting crystalline plastics (like PE or PP). This type of nozzle is most commonly used to manufacture packaging featuring a short cycle time and where colour changes are infrequent. In both designs the melt flows via semicircular channels located in the nozzle face. 

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