食品包装用聚丙烯薄膜中溶剂残留机理研究

。According to the random inspection results from National Supervision and Inspection Center, solvent residual problems of the food packaging film is very serious, which has so severely restricts development of food packaging industry and successful implementation of new standard GB/T10004-2008. Food safety and people's health could not be guaranteed greatly,food packaging safety is the premise to ensure food safety,so it is now imminent to study the solvent residual mechanism and relationship between content of residual solvent with microstructure of various materials used to make food packaging film. Polypropylene (PP) is often used to make printing and heat sealing layer in the composite flexible food packing film because of its good printability, heat sealing and high cost performance. When PP is printed or laminated with other plastic film, not only ink and adhesive used contains organic solvent about 50%, but also a large other amount of organic solvent as diluting agent is added, the adsorption of PP to these organic solvent is very high, so the content of residual solvent in PP films is high, which greatly affects the food safety. In order to provide a theoretical basis for solving solvent residual problems, in our study, the effect of the microstructure (crystallinity, crystal type and size), free volume and its influencing factors (glass transition temperature, nano-modification), affinity of the organic solvent for PP on residual solvent of PP will be studied, On that basis , the relationship model between the adsorption - diffusion coefficient of solvents in PP films and residual solvent of PP will be established under non equilibrium state. In the end, the key influencing factors of residual solvent of PP will be found and the mechanism of reducing solvent residual will be revealed.

据国家质检中心抽查显示，我国食品软包装溶剂残留问题严重，制约行业发展、威胁食品安全和人们身体健康、新标准GB/T10004-2008实施受阻。食品包装的安全是保证食品安全的前提,因此,对软包装材料的原料薄膜进行微观结构和溶剂残留机理研究迫在眉睫。聚丙烯(PP)材料具有良好的印刷适性、热封性和高的性价比，常被用作软包装的印刷层和热封层。PP薄膜在被印刷或与其它原料膜复合时，使用的油墨和粘结剂中含有50%左右的有机溶剂，同时还需要大量的稀释剂。这些有机溶剂在PP薄膜的中的残留值很高，为此本课题从PP薄膜微观结构出发，研究非平衡态下薄膜聚集态结构(结晶度、晶型、晶粒尺寸)、自由体积及其影响因素(玻璃化转变温度、纳米改性)、PP-溶剂的亲和性对溶剂残留的影响规律，建立吸附-扩散系数与溶剂残留量的数学模型，找出影响溶剂残留量的关键因素，提出降低溶剂残留量的机理，为提高包装材料食品安全性提供理论依据。

在复合软包装中，聚丙烯薄膜是主要的印刷基材和热封层的原料。普通聚丙烯薄膜的结晶度低，无定形比例大，与乙酸乙酯等常见的有机溶剂的亲和力大，因此与聚酯和尼龙薄膜相比，印刷或复合后的聚丙烯薄膜具有较高的溶剂残留值。本项目旨在揭示聚丙烯薄膜微观结构（结晶度、晶粒尺寸、玻璃化转变温度、自由体积分率和孔径分布）与溶剂分子在薄膜中的溶解-扩散性质（扩散系数D和溶解度S）之间的规律，为设计生产更加安全的聚丙烯薄膜提供理论依据。研究发现在纳米二氧化硅存在下，聚丙烯经过单轴拉伸后得到的CPP薄膜中，首次发现具有蝌蚪外形结构（S-C结构）的晶粒，数量众多而体积较小的S-C增加了CPP的结晶度，使溶剂在薄膜中的扩散路径的曲折度增加；在被纳米二氧化硅改性的双轴拉伸薄膜（BOPP）中发现了横向和纵向束状纤维结构，它们相互交织成网状，在BOPP表层薄膜的最外侧，形成一个致密的壳层，阻止了溶剂分子向薄膜深处的扩散；改性薄膜中这些微观结构的变化，增加了力学性能、阻隔性能、热稳定性，降低了溶剂分子在薄膜中的扩散和溶解。聚合物在拉伸条件下形成shish-kebab串晶的过程中，原来的片晶结构被完全破坏然后形成重新排列结晶，还是片晶直接沿外力方向取向直接参与新结晶结构，一直是高分子物理争论的焦点之一，通过我们的研究支持了第一种观点。