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Monday, February 23, 2015

Heat Shrinkable Materials

Heat Shrinkable Materials Technology

     High energy ionizing radiation provides an economic process to crosslink specific polymeric compounds which can then be converted to heat shrinkable tubing, molded shapes or wrap around products.  The phenomena of heat shrink ability is best explained by understanding that a typical thermoplastic consists of crystalline and non-crystalline phases.  When a thermoplastic is heated above the crystalline melting point, the polymer softens and flows readily.  However, if this same polymer is subjected ti ionizing radiation, crosslinks form between the individual polymeric molecules.

     When the crosslinked polymer is heated above the crystalline melting point, the crystalline areas "melt" but the polymer itself does not flow since it possesses form stability due to the presence of the crosslinks.   At this stage the polymer is similar to an elastomer.   If the shape of this heated material is changed, it will return to its original shape.  That is, it will remember the form it possessed at the time it was crosslinked.  However, if this polymer is held in a new shape while above its melting point, and then is allowed to cool in this condition, crystalline areas will reform.  crystalline forces will then keep the crosslinked polymer from returning (shrinking) to its original crosslinked shape. 

     Once the polymer is reheated above its melting point, thereby eliminating the crystalline forces, the elastomeric forces will cause it to recover back to its original crosslinked shape.  The term "elastic memory" is used to describe this phenomenon. 

     When the object to be covered is placed inside the heat shrinkable tubing and the tubing is heated, it will shrink around and conform to the shape of the object as it attempts to return to its original crosslinked shape.  Therefore, heat shrinkable products are useful in covering objects whose size is between the expanded diameter (the supplied size) and the original crosslinked diameter which is the freely recovered diameter of the tubing. 

     Since heat shrinkable polymers are crosslinked, they do not melt and flow.  They, therefore, have useful properties above their melting point. Because these heat shrinkable polymers often operate at elevated temperatures, only selected nonvolatile antioxidant systems can be utilized.  Also, since ionizing radiation can render most antioxidants ineffective, antioxidant technology had to be advanced in order to establish superior long-term thermal stability properties. 

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