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Flat-lay panels are widely used in furniture, doors, flooring, wall panels, and ceilings. In recent years, the popularity of minimalist, lightweight luxury, the prevalence of integrated quick-installation systems, and stricter environmental regulations have accelerated the development of PUR flat-lay technology in the home furnishing industry. Flat-lay materials are constantly being updated, and different combinations of substrate and film require different PUR materials. The saying "30% glue, 70% craftsmanship" emphasizes the importance of process control for high-quality gluing. However, improper glue selection can make process control extremely difficult, impacting gluing quality and production efficiency. Therefore, the correct glue selection can prevent many production problems at the source and ensure a higher process safety factor.
Many users prefer low-priced PUR for cost competitiveness, but others prefer premium imported brands for quality assurance and marketing appeal. Different PURs have varying applicability for different materials and working environments, and performance doesn't necessarily correlate with cost. Choosing the most suitable PUR from the numerous PUR products on the market requires a thorough understanding of materials, equipment, workshop conditions, and process requirements. This allows for a scientifically selected adhesive to ensure that the chosen PUR product meets all requirements and minimizes risk.
Table 1 Common material types and characteristics of flat-laid panels in the home furnishing industry
Material
As customization becomes more prevalent, the variety of materials used in the home furnishing industry is expanding. Flat-lay board materials, initially dominated by plain wood boards, have gradually evolved to include melamine-coated, metal, wood-plastic composite, stone-plastic composite, and inorganic materials. Films, initially dominated by high-gloss films, have gradually evolved to include thin PVC, fireproof panels, paper, veneer, cloth, aluminum foil, and galvanized sheet. Furthermore, the market is seeing an increasing number of difficult-to-bond materials, such as films with surface energies around 30 mN/m and PVC foam boards. These new variables complicate the correct selection and use of PUR.
a. Material adhesion
b. Surface effect
High-gloss film aims for a mirror-like finish, so PUR with low viscosity and good ductility is recommended, especially for thinner films. For high-gloss films laminated to airtight substrates, users without air conditioning in the summer can consider products with high initial tack. PUR with high initial tack typically also has high viscosity, which can be compensated for by using a sweeping roller to achieve a better high-gloss effect. For transparent PET film laminated to triamine board, while meeting the requirements of conventional high-gloss films, the adhesive layer must be transparent and highly resistant to yellowing, and must be free of visible impurities and bubbles.
c. Bubbling caused by airtight materials
Wood-based panels are mostly porous, providing a channel for CO2 release and rarely causing bubbling. If both surfaces are impermeable, PUR must be formulated with low CO2 release and high initial adhesion. Special care is required with plywood panels. Although the surface is porous, the continuous adhesive film between the veneers has poor air permeability. If the adhesive is not selected correctly, bubbling can often occur in the summer. It is important to note that the air permeability of wood-plastic panels and films varies. Therefore, it is sometimes found that under the same process conditions, certain substrates and certain films are more prone to bubbling. Metal-to-metal composites require special attention. Lack of moisture in the adhesive layer in the middle may prevent it from curing, creating a bubbling risk during end-use.
d. Surface particles caused by substrate
e. Large deformation of the membrane causes rebound
equipment
There are three main types of roller coating systems used in flat lamination production lines: conventional three-roller gluing machines, gluing machines with sweeper rollers, and anilox roller gluing machines. Some customers, such as those applying aluminum veneer film, also utilize blade coating systems. There are also three types of pressurized lamination systems: rubber rollers with steel rollers, continuous roller presses, and cold presses. To ensure quality, high-end production lines are equipped with static eliminators, effective preheating systems, online corona machines, or primers. The choice and configuration of user equipment primarily depends on the material. When selecting glue based on certain characteristics of the equipment, the following factors should be considered:
a. Due to aging equipment, the temperature of the glue roller cannot be increased, making it difficult to reduce the amount of glue applied with higher viscosity, which not only increases costs but also affects the surface effect. Therefore, it is necessary to use PUR with lower viscosity;
b. Anilox roller equipment is generally not used to produce high-gloss films, and the footboard is prone to glue leakage, so PUR with too low viscosity should not be used;
c. If the glue application port of the roller coating system is not close to the glue baffle, the glue at both ends of the glue coating roller will be easily aged and stringy after being heated for a long time. It is necessary to choose PUR with good thermal stability;
d. Users who have a cold press and can accept pressure retention can consider choosing PUR with lower initial adhesion and higher overall cost-effectiveness;
e. Production lines with no preheating system or poor preheating effect need to choose PUR with good low-temperature wettability.
Workshop conditions
Workshop conditions vary from user to user. Some users will build insulation rooms to ensure relatively stable temperature and humidity conditions; some users' workshop conditions are very simple, and the temperature in the workshop may fluctuate between 0 and 45 degrees within a year.
For users with large temperature swings in their workshops, a successful adhesive test at one point in time doesn't guarantee future compliance. Therefore, extreme temperature conditions must be considered when selecting PUR. Furthermore, considering material and equipment variability, it's generally difficult for a single PUR product to withstand such a wide range of temperature conditions. When selecting adhesive, consider at least two versions, winter and summer, to meet the basic requirements of a long open time in winter and high initial tack in summer. Switching between them must also be done at the appropriate time.
b. Undoubtedly, users with stable workshop temperature and humidity have more options for PUR adhesives and can theoretically use more cost-effective PUR products, thus having a greater competitive advantage overall.
Process requirements
Even when producing similar products for the same buyer, different users often have different processes. Some are limited by equipment and workshop conditions, some seek efficiency or cost reduction, and some are driven by operational habits. Special processes often have specific requirements for certain PUR properties.
a. Insulation rooms are usually closed spaces in winter, and the exhaust system is not turned on for heat preservation. Therefore, the smoke and odor generated by roller coating over a large area exposed to heat are easily accumulated. Low-odor and environmentally friendly PUR is more popular with operators.
c. The customer does not stop the machine during lunch break, so PUR with good thermal stability is required. Even in the rainy season with high temperature and humidity, the equipment can still maintain stable viscosity and evenly apply glue after idling for one hour.
Based on the above analysis, every detail regarding materials, equipment, workshop conditions, and process plays a crucial role in adhesive selection. Choosing the right adhesive is crucial for smooth production and final product stability. Using expensive adhesives doesn't guarantee safety, while using too cheap adhesives often makes product stability difficult to guarantee. Only by scientifically selecting the most suitable PUR after professional analysis can users maximize their competitiveness.
This article is excerpted from the March/April 2021 issue of Asian Panels and Furniture.
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Editor/Sharon