Why Poor Adhesion Occurs in Low-Temperature Decal Processes and Glazing on Glass Bottle Bottoms

17 10 月, 2024

Author: Roe Li

The glazing on the bottom and low-temperature decal processes for glass liquor bottles are the most common deep processing techniques in the entire glass packaging industry for spirits. Their characteristics include a variety of vibrant colors, and the decorative paper does not discolor or turn dull. However, the downside is that the adhesion of the decal or glaze is unstable.

This blog aims to dive deeper into the causes of poor adhesion in these processes and explore possible solutions. We will discuss the impact of various factors, including materials, production techniques, and environmental influences; while offering insights that can help glass bottle manufacturers and decorators address these challenges more effectively.

Table of Contents

Conventional Deep Processing Production Techniques
Special Deep Processing Production Techniques
Low-Temperature Decal Production Process
Other Factors Affecting Decal Adhesion
Improvement Directions

Conventional Deep Processing Production Techniques

Conventional Production Process Flow

The conventional production process involves first applying glaze to the bottom, followed by decal application and firing. This process is considered conventional mainly due to its reasonable and simple sequence, high production efficiency, and relatively low production costs. However, it also has drawbacks, particularly a high requirement for the quality of the decorative paper. To meet the production standards, it is crucial to ensure stable adhesion of the paper.

This process is commonly used in the production of various glass bottles, including beer bottles, whiskey bottles, liquor bottles, perfume bottles, and even milk bottles. For glass bottle manufacturers, achieving consistent adhesion is essential to maintain the aesthetic appeal and durability of the products. The technique also plays a key role in the quality control of glass jars and other containers used in packaging. The stability of the decal and glaze adhesion directly impacts the overall marketability and performance of these bottles, making it a critical focus area in the glass packaging industry.

Analysis of Factors Affecting Adhesion Stability in the Process

In this process, the bottom of the glass bottle is first glazed, and before applying the glaze, a PE rubber sleeve (fixture) is used to cover areas that do not require glazing, leaving the bottom exposed. After glazing, the bottle is baked at around 200°C（ 392°F.）for 20-30 minutes to dry the glaze to more than 90%. However, during the baking process, high temperatures cause organic substances in the PE sleeve to volatilize onto the surface of the glass bottle, which leads to reduced adhesion of the decorative paper after the decal is applied. The extent of volatilization depends on the material of the PE sleeve, its usage duration, and the baking temperature. The longer the PE sleeve is used, the more it ages, releasing more organic substances and causing a greater impact on the adhesion of the decorative paper to the glass bottle surface.

Special Deep Processing Production Techniques

Special Production Process Flow

The special production process involves applying decals, baking them, and then glazing the bottom. This process is considered special due to its complex and repetitive sequence, resulting in relatively higher production costs and lower efficiency.

The advantage of this process is that it ensures strong adhesion of the decorative paper, making it a reliable choice for high-end glass packaging, including liquor bottles, perfume bottles, and even glass jars. This enhanced adhesion quality is particularly beneficial when producing glass containers used for premium products such as whiskey bottles and alcohol bottles, where aesthetics and durability are crucial. However, the downside is that the adhesion of the glaze on the bottom of the bottle is harder to control and tends to be unstable. The challenge in managing the glaze adhesion becomes more apparent in larger production batches, affecting both the visual appeal and functionality of the bottles. This instability can also impact the overall quality of products, leading to potential issues during storage or transportation.

Analysis of Factors Affecting Adhesion Stability in This Process

In this process, decals are applied and fired first, followed by glazing the bottom. Since the glass bottle surface is not affected by the PE rubber sleeve, it remains clean, and the adhesion of the decals and the baking process is very stable. However, during the decal application, the water from the decorative paper can flow to the bottom of the glass bottle. As the water contains viscous substances, it is difficult to fully clean the bottom after it flows there, affecting the adhesion of the glaze after spraying.

Additionally, the glazing line’s built-in oven is relatively short, resulting in insufficient baking time (the glaze dries to about 90%). To fully cure the glaze and improve adhesion, the bottle needs to be placed in the decal oven again, which slows down the process. The repeated baking can also cause issues such as the loss of toughness in the decorative paper, cracks in the hot stamping, and other problems. It was moreover, repeated handling increases the likelihood of human error, which may lead to defects such as bottle collisions or scratches.

Low-Temperature Decal Production Process

Analysis of Low-Temperature Decal Ink Properties

The main factor affecting the adhesion of low-temperature decals is the ink used for printing the decorative paper. There are three types of ink used for low-temperature decals:

UV Ink (Ultraviolet Light Source)

LED Ink (Cold Light Source)

Solvent-based Ink

Each type of ink has distinct properties and impacts adhesion differently, which is crucial for ensuring stable and durable decal application.

UV Ink (Ultraviolet Curing Ink) refers to ink that cures under ultraviolet light at a specific wavelength. When exposed to UV light, the photo initiator in the ink absorbs photons, converting them into free radicals. These radicals initiate polymerization, causing the prepolymer and photosensitive monomers to transition into an excited state, generating charge transfer complexes. This cross-linking process quickly solidifies the liquid ink into a solid state, completing the curing. UV ink is less commonly used by decal manufacturers due to its high energy consumption, cost, and environmental concerns (such as VOC emissions). This type of ink might be seen in high-end glass bottles, such as perfume bottles or premium liquor bottles, where detail and clarity are paramount.

LED Ink operates similarly to UV ink in terms of curing principle, but the difference lies in the light source used. While UV ink relies on a heat-based light source (which requires preheating and generates ozone), LED ink uses a cold light source for curing. This results in immediate curing without ozone production, and the power consumption of LED ink is about one-fourth that of traditional UV ink. This makes LED ink a more energy-efficient option, particularly for mass production of glass jars or alcohol bottles that require a faster turnaround.

Solvent-based Ink is a more traditional type of ink, which is based on solvents and cures through the evaporation of these solvents. The curing process is slower, which can significantly impact the production efficiency of decals. However, the advantage of solvent-based ink is that the decals printed with it tend to have better flexibility and toughness after firing, making them less brittle. This flexibility is particularly beneficial when used on glass bottles such as milk bottles, where strength and durability are key for both aesthetics and functionality. In low-temperature decal printing, a combination of LED ink and solvent-based ink is often used. For example, the process of printing a decorative paper using these inks may involve the following steps:

In this process, the more LED ink used, the more times the ink undergoes light curing, which makes the decorative paper more “brittle” and reduces its toughness. The level of brittleness is influenced by the number of layers printed. That is, if solvent-based ink is used exclusively in the decal printing process, efficiency and production capacity are significantly low, making it impossible to shorten delivery times. On the other hand, if LED ink is used exclusively, the printing speed is very fast, and production capacity is high, but the paper becomes more brittle and prone to damage. During production and filling, this can lead to the decorative paper falling off easily. Therefore, decal manufacturers generally use a combination of LED ink and solvent-based ink to balance efficiency, toughness, and production speed.

Analysis of Poor Adhesion in Low-Temperature Decals

The poor adhesion of low-temperature decals on clear glass bottles is a significant challenge in the entire low-temperature decal industry, particularly concerning the decals’ limited water resistance. The adhesion of these decals is influenced by the base oil used in the paper, which often contains silicone oil. In humid environments, the silicone oil undergoes a process known as “silicone hydrolysis,” leading to a decrease in adhesion strength. This means that, over time, especially during storage or production, the decals’ adhesion tends to deteriorate, with humidity being a major factor. This issue is especially prevalent during the warmer, more humid months, such as late spring and early summer, which can cause significant problems for decal manufacturers and glass bottle producers alike.

Other Factors Affecting Decal Adhesion

Baking Temperature: The baking temperature significantly impacts the glossy coating of the decal. When the temperature is too high or the number of baking cycles is excessive, it can cause the glossy coating to become brittle, thereby reducing the decal’s overall toughness and adhesion. This brittleness can be particularly problematic for glass bottles, including whiskey bottles and perfume bottles, as the fragile decal may crack or peel away easily, affecting both the aesthetic and functional integrity of the product.

Peelable Film for Low-Temperature Baking Products: If the peelable film is removed before the baking process, it can compromise the adhesion of the decal, leading to poor bonding between the decal and the glass surface. This can be particularly problematic for products like liquor bottles and beer bottles, where clarity and durability of the label are crucial to brand representation.

Raw Materials and Process Variations: Differences in the raw materials used for the decals—such as variations in the ink, paper, or adhesives—and inconsistencies in the decal manufacturing process can also impact the adhesion properties. For example, the choice of solvent-based or LED ink for decoration on glass jars or spice jars could influence how well the decal stays adhered over time.

Improper Application During Decal Application: If the water from the decal is not fully squeezed out or the decal is soaked for too long, it can lead to improper application. This improper application can result in air bubbles or wrinkles in the decal, significantly reducing adhesion. This is especially relevant when decorating products like mason jar sizes, candle jars, or even soda bottles, where smooth, uniform adhesion is essential for a high-quality finish.

Improvement Directions

Existing Process Improvement

The decal supplier will adjust both the raw material properties and the production process of the decals. By collaborating with their upstream partners (such as ink suppliers), they will develop more adaptable inks that enhance decal adhesion. The production and printing process will also be optimized, using the best ink combination to improve decal flexibility.

Testing on the newly formulated ink and adjusted printing process has already shown improved adhesion and flexibility, but further batch tests are required to ensure stability and assess changes in storage quality over time.

In addition, the process will switch to applying the decal and baking it before spraying glaze on the bottom of the bottle. The lifespan of the PE rubber sleeve used in the glazing process will also be reduced from 500,000 uses to 400,000, with an additional cleaning after 200,000 uses to minimize the release of organic substances. The surface of the glass bottle will also be cleaned before glazing to ensure proper adhesion of the glaze on the bottle’s bottom.

The previous baking temperature for the decals was uniformly set between 185°C and 190°C (365°F–374°F). Moving forward, the optimal baking temperature will be determined through small batch tests based on the supplier’s specific decals before mass production is implemented.

Lastly, the process of removing the peelable film will be moved to after the baking step, preventing decal misalignment or damage during removal before baking, which could affect adhesion.

Innovative Process Improvement

Due to the instability factors in the decal process and risks associated with using PE covers during glazing, the use of PE covers will be eliminated. The traditional “bottom glazing + decal application and firing” technique will be replaced with “bottom gradient glazing + clear varnish spraying + decal application and baking.” This change aims to reduce external factors that affect decal adhesion on glass bottles, including liquor bottles, whiskey bottles, and glass jars. Once customer approval is received, this process will be implemented to improve decal adhesion and overall product quality.

Innovative Decal Adhesion Techniques at Roetell Glass

Roetell Glass, with over 39 years of expertise in glass packaging, specializes in providing high-quality glass bottles, jars, and custom packaging solutions. Our advanced production techniques and commitment to innovation ensure we meet the diverse needs of industries such as liquor, perfume, beverage, and food packaging. By leveraging cutting-edge technologies and maintaining strict quality control, Roetell Glass delivers durable, aesthetically pleasing glass products, including whiskey bottles, beer bottles, and glass jars. Our team continuously seeks ways to improve processes, such as the new decal adhesion methods, ensuring we remain at the forefront of the glass packaging industry. Roetell Glass is dedicated to enhancing product quality and production efficiency, which is why we are constantly refining our decal adhesion methods. Our new approach focuses on eliminating the uncertainty associated with traditional PE covers during glazing, opting for bottom gradient glazing and clear varnish spraying. This innovation not only improves the adhesion of decals on glass bottles but also minimizes the impact of external factors, ensuring that our glass products maintain their vibrant, high-quality finishes. By continually advancing our decal adhesion techniques, Roetell Glass aims to offer superior, durable packaging solutions to meet the ever-evolving demands of the market.

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