Packaging Knowledge: A Brief Discussion of Four Types of Flexible Packaging for Milk

Milk is a very common food in our daily lives. But I wonder whether you have ever noticed the packaging formats of milk, especially flexible packaging formats. This article will provide a brief analysis of flexible packaging for milk, in the hope of inspiring friends who are preparing to enter the related industry.

Common flexible packaging for milk falls into two major categories: all-plastic flexible bag packaging and paper-plastic composite packaging. Each of these two major categories can be further subdivided into several different types. We will introduce them one by one.

Film Knowledge: A Brief Discussion of Flexible Packaging for Milk

Pure PE black-and-white film

Pure PE black-and-white film is the earliest form of flexible packaging for milk. It is generally produced by a three-layer co-extrusion blown film process. Depending on the equipment, five-layer or even seven-layer co-extrusion blown film structures can also be used. As the name suggests, the outer side of this film is white and the inner side is black. The purpose of this design is to allow surface printing on the white outer background; at the same time, the black inner side provides a certain light-blocking effect, extending the shelf life of milk. Milk is a high-value content, so the packaging has relatively high strength requirements. Therefore, 1MI linear resin is often used with a small amount of low-melt-index high-pressure resin, and a certain amount of metallocene is added to the heat seal layer to ensure heat seal performance.

EVOH co-extruded black-and-white film

Compared with pure PE black-and-white film, EVOH co-extruded black-and-white film incorporates an EVOH barrier layer. Therefore, at least a five-layer co-extrusion structure is required for production. In fact, many manufacturers are already using seven-layer co-extrusion structures to optimize the functional design of each layer's formulation. The selection of adhesive resin is a key consideration in the formulation design of EVOH co-extruded black-and-white film. Improper selection of adhesive resin can lead to insufficient EVOH adhesion and delamination, and may also cause water ripple problems. Compared with pure PE black-and-white film, the barrier properties of EVOH co-extruded black-and-white film are greatly improved. Therefore, it is often used for milk packaging in summer and autumn when temperatures are higher.

The gable-top carton gets its name from its slanted top that resembles a roof. This milk packaging looks attractive, displays well on shelves, and is easy to open. The packaging material structure is simple, generally PE-coated paperboard. However, because the PE coating process requires high processing temperatures, low-odor coating PE needs to be selected for production. Since it has no barrier properties, the gable-top carton is only suitable for pasteurized milk that requires refrigerated storage.

Tetra Pak is a rapidly developing milk packaging format in recent years. It is a paper-aluminum-plastic composite structure packaging invented by Tetra Pak. According to shape, it can be subdivided into Tetra Fino, Tetra Brik, Tetra Prisma, etc. As Tetra Pak's patents expired, a large number of domestic Tetra Pak-style packaging manufacturers have emerged in China in recent years. The packaging materials for Tetra Pak are all processed in one pass through a multi-head co-extrusion coating process.

Step 1: Through LDPE coating lamination, the printed paper is bonded to aluminum foil to obtain a "paper/LDPE/aluminum foil" structure;

Step 2: On the aluminum foil side, EAA/heat-seal PE is co-extrusion coated to obtain a "paper/LDPE/aluminum foil/EAA/heat-seal PE" structure;

Step 3: On the outer side of the paper, LDPE is coated to protect the outer printing, obtaining the final product structure of "LDPE/paper/LDPE/aluminum foil/EAA/heat-seal PE".

Since the back seal of Tetra Pak is a lap seal structure, a special heat seal strip is required during the packaging process to enhance the strength of the back seal.

Combibloc was invented and promoted by SIG. The packaging material structure of Combibloc is similar to that of Tetra Pak, so in theory it can be produced on the same multi-head co-extrusion coating equipment. However, generally speaking, Combibloc uses thicker paper, and due to the different heat sealing method, Combibloc has different requirements for heat-seal PE. Combibloc's heat sealing is achieved by blowing hot air onto the heat-sealing surface, so it has higher requirements for the low-temperature sealing performance of the heat seal layer. Therefore, the heat-seal PE for Combibloc generally needs to use polyolefin plastomer to meet the requirements.