What is the discussion on the composite process of BOPP and CPP

BOPP//CPP is the most widely used laminated structure for light packaging in the flexible packaging industry. This structure is simple, easy to operate, and suitable for packaging dry foods with a short shelf life, such as bread and snow crackers. Some food manufacturers also use it for gas-flushed packaging to extend the storage time of the contents.

However, although the lamination process for this structure appears simple, it faces many challenging issues: for example, very low peel strength; severe degradation of peel strength; lamination bubbles; poor heat seal strength leading to easy bag breakage, etc.

Based on lamination experiments, material analysis, and practical application experience, several suggestions are proposed for the lamination process of this structure.

I. Characteristics of the BOPP//CPP Process

BOPP is biaxially oriented polypropylene. It is produced by first forming a high-molecular polypropylene melt into a sheet or thick film through a narrow die, then stretching it simultaneously or sequentially in two perpendicular directions (longitudinal and transverse) at a specific temperature and set speed in a specialized stretching machine, followed by appropriate cooling, heat treatment, or special processing (such as corona treatment, coating, etc.). CPP film involves a similar process, is cheaper than linear low-density polyethylene, and has a lower density. Due to the inherent rapid cooling characteristics of cast film, it exhibits excellent smoothness and transparency, along with good water resistance.

Adhesives are divided into two types: one is solvent-based laminate adhesive. These are primarily: high molecular weight polyester-based polyurethane, with an added curing agent component; and high molecular weight polyether-based polyurethane, with an added curing agent component. The other type is solvent-free laminate adhesive, primarily a hybrid of polyether and polyester.

Based on the properties of material formation and the characteristics of adhesives, the following conclusions can be drawn:

Both BOPP and CPP are non-polar materials with good crystallinity.

Both materials require corona treatment during lamination application to achieve microscopic surface roughness.

In BOPP//CPP lamination, the adhesive functions through physical adsorption relying on a large surface area.

Adhesives are mostly polar materials and have poor wettability on non-polar material surfaces.

Due to poor surface wetting, the adhesive's own cohesion is relatively high, leading to even poorer adhesion to the materials.

Practical experience shows that when using solvent-based adhesives for this structure, it is often necessary to reduce the amount of curing agent to achieve a slightly tacky state, which provides better peel strength. Polyether-based adhesives generally offer higher strength than ordinary polyester-based adhesives. When using solvent-free lamination, the overall film roll tends to be stiff, and strength degradation is severe.

II. Experimental Demonstration

The solvent-free lamination process was selected for the experiment. The film used was BOPP20//CPP20 for snow cracker packaging, with an adhesive application rate of 1.3 gsm, using a Nordmeccanica Simplex SL. The Gaomeng adhesive system was chosen:

Table 1: Experimental Results

Conclusions:

For solvent-free lamination of BOPP//CPP, severe strength degradation occurs under the standard ratio.

Reducing the amount of curing agent can improve the strength of BOPP//CPP to some extent, but it cannot meet long-term degradation requirements.

Different adhesive systems result in basically no significant change in the degradation strength obtained.

Specially designed adhesives significantly improve the laminate strength for this structure.

III. Suggestions for the BOPP//CPP Lamination Process

Prevent corona treatment expiration. Due to the good surface smoothness of PP, prolonged storage can cause the corona treatment to expire, reducing laminate strength.

The area of ink printing significantly affects lamination. The adhesive ratio should be chosen according to the printed ink area.

The lamination speed should not be too fast. Due to the poor surface wettability of the raw materials, excessive speed can easily lead to lamination bubbles.

Different thicknesses of base film materials vary. BOPP 38µm is more prone to interlayer separation than BOPP 18µm.

For laminated film, peeling should occur in a slightly tacky state. The better the dryness, the worse the strength often is.

During heat seal testing for light packaging, the ideal tear state is rupture after the base film stretches. If delamination occurs in the laminate or the heat seal breaks, adjustments to the lamination process and bag-making process should be considered.