melting temperature
LDPE is typically processed at melt temperatures of 600 to 630°F. These high temperatures are necessary to promote surface oxidation of the resin and ensure adequate adhesion to the substrate.
However, it is not necessary to oxidize the acid copolymer or ionomer, since the acid functionality is responsible for the adhesion to the foil or paper. These resins should be processed at the lowest melt temperature consistent with adequate product performance. An obvious reason for choosing a lower melting temperature is to reduce the chance of odor or taste issues when packaging sensitive products. Another reason is shown in Figure 21.8. Both acid copolymers and ionomers can undergo a process called anhydride crosslinking, in which carboxyl groups in adjacent molecules react to form anhydride crosslinks and produce water as a by-product. This reaction occurs at a significant rate within the temperature range in which copolymers and ionomers are normally processed. The cross-linking reaction leads to gel formation, while the water causes mold build-up problems. As an approximate guide, the melt temperatures shown in Table 21.2 are recommended for acid copolymers and ionomers, assuming that the desired product properties can be achieved under these conditions.
Figure 21.8 Anhydride crosslinking
other considerations
Both acid copolymers and ionomers are slightly corrosive, so metal parts that come into contact with these products at high temperatures should be plated with nickel or chromium. Another factor to consider is the moisture sensitivity of ionomers. Ionomers are hygroscopic and are therefore packaged in foil-lined bags and boxes. When opening the box, only a corner of the lining should be cut off to allow insertion of the delivery tube. When done right, it's easy to reseal the lining of a partially used box by folding it over and taping it in place. With normal precautions, the moisture sensitivity of ionomers presents no operational problems.
