Helping Your Products Weather the Elements
This article discusses how you can match pressure-sensitive adhesive films for outdoor use with the environments in which they'll be displayed.
Understanding the environmental conditions that your application may face is critical to selecting the best materials for a label, decal, or graphic job. That's because weathering can have a dramatic effect on the substrates, overlaminates, adhesives, and inks that make up your application.
Although any one environmental factor may not be overly detrimental, the combination of multiple conditions can have a significant impact on how long your application will function at its peak level of effectiveness. Exposure to ultraviolet (UV) light, humidity, or heat alone can impact product performance, and together these three elements can cause substantial deterioration to printed film products whose operational environments are primarily outdoors (Figure 1).
Choosing the right substrate, overlaminate, and adhesive can make the difference between the failure and success of the application. For example, the most common effects of UV degradation on pressure-sensitive films include chalking, cracking, gloss change, color change, edge lift, and loss in tensile strength. However, there are many different substrates available that can withstand UV light for extended periods of time. Vinyl and polyester films are only a few of the many potential choices here. Furthermore, it is critical to look beyond the label and graphic material when it comes time to produce your application. You could choose a film that will last for years outdoors, but if you select an improper ink, the graphic may fade within weeks.
A key factor in selecting the best materials for the job lies in determining what performance characteristics you require. For example, does the decal or display need to be removed intact, or does it need only to maintain its physical integrity in one place? Is some degree of hazing or fading of the film or print acceptable? Should the film have an adhesive that will adhere permanently, or should it be removable? Is the application surface rough or textured, flat or curved?
Life span also plays a role in material selection. If you the need the printed film to remain visually appealing for two years, you would pick a different material than one used for a two-week promotion. The desired life span also will influence adhesive and ink selection. Many types of substrates, adhesives, and inks are available that address all these issues, but testing the materials in various environments is the only way to ensure you're using the right products for the job. This article will provide an overview of the types of climatic conditions your products may face, the effects of these conditions, and methods of simulating the conditions in order to test product performance.
Damaging UV light
Have you ever gone to the beach and come home with a severe sunburn? If so, you were a victim of photodegradation caused by exposure to UV light. At its simplest, photodegradation is an alteration of molecular bonds that results in damaging effects to exposed surfaces, such as human skin and graphic films. Although UV exposure is only one cause of weathering, it commonly is the one that receives the most attention.
UV light is a segment of the electromagnetic energy spectrum. It is characterized by its wavelength and amplitude. Wavelength typically is measured in nanometers (nm), or billionths of a meter. Generally speaking, the shorter the wavelength, the more energy, and the more damaging the UV light can be. Both UV (which occurs between 295-400 nm) and visible light (which falls between 400-800 nm) can contribute to weathering.
The Commission Internationale de l'Eclairage (CIE), an international standards organization, has established the UV spectrum at 295-400 nm. However, some UV light does exist below that range. Fortunately, the ozone present in the stratosphere absorbs and eliminates most radiant solar UV energy below 295 nm.
The UV spectrum itself comprises three subranges: UVA (315-400 nm), UVB (280-315 nm), and UVC (280 nm and lower). Of the three, UVC is the most damaging. This component of the UV spectrum is germicidal and is commonly used for sterilization of medical instruments in hospitals. UVB is the range most responsible for your sunburn on the beach and is the most damaging energy that reaches the earth's surface. It is the band of UV light most often used for curing UV inks. UVA light, commonly known as black light, is the least damaging subrange, but it also can contribute to sunburn and cause photodegradation in graphic films.
A product can be tested for resistance to UV exposure and other weathering conditions either artificially or naturally. Artificial weathering attempts to accelerate the weathering process, and it allows the evaluator to select various conditions and exposure times. Two of the most common devices for artificial weathering are fluorescent and xenon-arc weatherometers. Materials also can be exposed to natural sunlight under accelerated weathering conditions or in real-time outdoor aging tests. The following sections detail each method.
Fluorescent UV weathering with fluorescent bulbs (Figure 2) is a widely used and economical test method. If desired, UV exposure can be integrated with heat and humidity, the relevance of which will be discussed later. On some models, users can change bulb types to approximate different situations. This ability allows users to compare the performance of different materials for specific applications, such as window signage that will be placed inside a storefront window or after-market interior auto decorations. One drawback to fluorescent testing is that no fluorescent bulb exists that matches the full UV spectrum of sunlight.
Xenon arc The xenon-arc weatherometer (Figure 3) is the best available means to artificially accelerate weathering, but it is more expensive to purchase and operate than fluorescent-UV equipment. On xenon-arc systems, the spectral output is a close approximation to normal solar lighting, and it is controlled by the use of borosilicate and/or quartz light filters. This output includes visible wavelengths of light and can easily simulate sunlight or sunlight through glass, depending on the combination of light filters used. Heat and humidity can be applied as well.
Accelerated natural light In this weathering method, materials are exposed to concentrated natural sunlight using a device such as EMMA (Equatorial Mount with Mirrors for Acceleration, a system developed by DSET Laboratories, Inc., Phoenix, AZ). Natural light is concentrated by the use of mirrors that focus the light onto the sample (Figure 4). The acceleration is slower than in the case of an artificial weathering test, but the exposure covers the entire UV and visable spectrum. The entire apparatus is mounted on a rack that tracks the sunlight through the course of the day, thus achieving maximum daily exposure. Humidity also can be applied on an EMMA.
Real-time outdoor aging Some customers believe the best proof of weathering resistance comes from testing a sample under actual conditions. In this case, a sample may be left outdoors in natural sunlight for years and monitored. If using this method, however, it is important to take both geography and test protocol into consideration. Labels and graphics will age differently when exposed to different extremes in temperature, humidity, etc. In addition, the amount of UV light exposure will vary if samples are exposed to the sun at different angles.
How UV exposure affects specific substrates
With UV-light exposure, the primary concern lies with the substrate and any overlaminate that may be included on the product. Substrates commonly used for functional outdoor graphics that will be exposed to UV light include pressure-sensitive films made of polyester, vinyl, Tedlar, and acrylic materials. Table 1 shows common characteristics of these materials, as well as applications for which they are suited.
Polyester provides long-term dimensional stability and excellent tensile strength when exposed to thermal conditions. It usually comes in white, as well as clear and clear matte, and can be dyed, textured, or metallized. Polyester is an ideal medium for supporting functional graphics, such as product-identification labels, operating instructions, outdoor-equipment labels, and warning/safety decals. Although polyester is durable and resilient, it can become brittle after long-term exposure to UV, making it difficult to remove intact. Some suppliers, however, offer polyester films with UV-blocking and absorbing additives that can help improve the life span of the materials and make them suitable for long-term labeling applications.
Tedlar and vinyl films can offer an even greater degree of durability. They are ideally suited for a range of graphic applications, such as decals and labels for lawn or garden tools, heavy-duty automotive or industrial equipment, and even outdoor toys for children. These products offer the highest level of substrate performance. Vinyl, for example, can conform to complex curves and is less likely to become brittle than polyester. Most suppliers also offer a range of acrylic films that, like most acrylic adhesives, hold up well under exposure to UV light. However, acrylics tend to be more brittle and less flexible than vinyl.
In terms of protecting substrates, UV varnishes and overlaminates are considered effective methods. Printable films that are less UV-durable, such as polypropylene, can have their durability extended by the use of an overlaminate that provides UV protection. Such overlaminates include clear vinyl and polyester films that also can be coated with UV-absorbing adhesives. In addition, some suppliers have developed special materials that contain UV inhibitors to meet the challenges associated with product-identification and safety-hazard warning labels. These materials offer a cost-effective option for producing labels and decals with extended outdoor life.
Although substrates and overlaminates are the primary focus of UV protection, adhesives also must be considered. Many rubber-based adhesives, for example, do not hold up well to direct UV exposure. The adhesive can become brittle, leading to edge lift or adhesive failure. However, when used with an opaque or UV-blocking substrate, a rubber ad-hesive may be suitable for some outdoor applications.
|Table 1 Films for Outdoor Applications|
|Polyester (PET)|| Dimensionally stable |
Thermally and chemically resistant
| Material-tracking labels |
Safety, hazard, and instructional labels
Two-way window decals
|Vinyl (PVC)|| Flexible |
| Product-ID labels |
Safety, hazard, and instructional labels
|Acrylic|| Non-yellowing |
Highly outdoor durable
| Tamper-evident applications |
|Tedlar (PVF)|| Highly resistant to chemicals and environmental conditions |
Highly outdoor durable
| Product-ID labels |
Safety, hazard, and instructional labels
Labels for harsh environments
High humidity and application life
Humidity in and of itself is not as damaging to a pressure-sensitive film product as humidity combined with exposure to UV light. However, excessive humidity can cause some adhesives to haze over or whiten, altering the appearance of the film and printed graphics, particularly on clear film substrates.
The impact of humidity is a critical concern when the application must endure extremes in weather. For example, an outdoor graphic may be exposed to rain, snow, and high humidity during the course of its lifetime. This means the printer must take into account the application's ultimate geographic placement, because an adhesive that works well in the arid Southwest may not be suitable for the muggy Southeast or the cool damp Northwest. Geography is a major consideration when selecting the best adhesive for an outdoor application.
Material selection also should take into consideration the duration of promotion or desired life of the decal or label. For example, a promotional floor graphic that needs to last for a temporary outdoor promotion may not need the same adhesive system as a product-identification label applied to an all-terrain vehicle. The adhesive to use also may be dictated by whether the graphic film must stay attached permanently or needs to be removed later. The goal is to achieve the right balance of adhesive properties (i.e., sheer, tack, and peel) to satisfy the application requirements. Working with an experienced supplier who can offer a wide range of adhesives is the best way to ensure that your needs are met.
Heat is another significant concern in terms of a graphic film's weatherability. Excessive heat can lead to substrate shrinkage, which can cause the edges of a label or decal to curl or pull back from the surface. Substrate shrinkage also can leave adhesive behind, which can collect dust and dirt and create an unattractive halo. Additionally, heat can cause adhesives to yellow and lose some of their inherent adhesive properties. In a worst-case scenario, heat can reduce the effectiveness of an adhesive to the point that the label lifts, flags, or falls off the substrate surface.
As with humidity, understanding the ultimate environment that the application will face is critical. A durable-goods product label tailored to meet the high temperatures of Arizona may not function as well in the cooler climate of northern Maine. Of more concern are extreme fluctuations in the service temperature. Labels that move from high temperatures to low temperatures and back again may crack as a result. In addition to service-temperature concerns, you need to be aware of processing-temperature issues that may be influential in the graphic-production process. Certain polyester substrates will retain their dimensional stability, even if they are exposed to temperature extremes in processing. Some vinyls become distorted under the same conditions.
Beat the elements
Determining the weatherability and durability of a graphic film for outdoor use can be a complex undertaking. It involves careful selection of a substrate, adhesive, and overlaminate. You also must select the correct test method or accelerated-weathering device so that you can accurately assess materials in a time-frame that meets your needs. Look to your film supplier for guidance in selecting materials for specific environmental conditions and choosing the best methods to ensure those materials meet your weatherability requirements.
Bulb Specifics for Fluorescent Weathering Systems
Fluorescent weathering systems are produced by several manufacturers. Some models allow users to change bulbs to emulate different UV-exposure intensities. For example, the QUV Accelerated Weathering Tester, from Q-Panel Lab Products, Cleveland, OH, offers three bulb options. Each bulb is designated with a number that represents the peak emission of UV light that the bulb simulates. Combined, these three bulbs simulate a substantial portion of the UV spectrum:
* UVA-340: The light produced by this bulb has a peak emission of 340 nm. It provides UV light that most closely follows the spectral curve of natural sunlight.
* UVA-351: This bulb, with a peak emission of 351 nm, simulates daylight as it would appear through window glass, which filters out some UV wavelengths. It is used when testing win- dow-graphics materials that will be placed on the inside of storefront windows.
* UVB-313: This bulb has a peak emission of 313 nm and exposes a sample to a harsher envi- ronment than either of the other bulbs. The major drawback to using UVB-313 bulbs is that they produce an output below the normal solar cut off, thereby exposing a sample to conditions it would never experience in real-world applications. Hence, results can be anomalous.
One of the most important factors to bear in mind when utilizing a fluorescent-UV testing system is that the bulbs are limited to simulating the UV spectrum. They do not take into account any potential effects caused by exposure to the visible spectrum of light, which can have a significant impact on test samples.
Glossary of Defects Caused by Weathering
Chalking This describes the formation of a powder residue on films that is caused by oxi-dation. As the film reacts to the UV light, the untreated film breaks down, producing a chalk-like remnant.
Checking Checking refers to fractures in a substrate that manifest themselves as random breaks in the film surface. The fractures result from diminished tensile strength in the material.
Cracking This phenomenon can occur as a result of UV exposure or label or graphic shrinkage. It manifests itself as random breaks in the film's surface.
Crazing A level of cracking or checking that is visible only on the surface of the label or graphic.
Delamination Delamination is a separation of laminated layers of material. There are different degrees of delamination, ranging from edge lift (where disproportionate shrinkage between the overlaminate and base film result in a curling of the edges of the graphic or label) to complete delamination, where the layers split away from one another.
Embrittlement This defect occurs when a film loses its tensile strength due to weathering. The film becomes less flexible and more subject to cracking or tearing.
Fading Fading is a gradual decrease in the intensity of color.
Gloss loss A glossy surface may become progressively duller when exposed to UV light, heat, or humidity.
Hazing This is a loss of the overall clarity of a sample. The sample begins to develop a white hue, impeding the attractiveness of the graphic or label. Unlike chalking, which occurs on the surface, this reaction occurs within the film itself as a result of exposure to UV light.
Shrinkage The loss of dimensional stability due to outdoor exposure. It can result in an adhesive halo.
Yellowing The tendency of some films to degrade with exposure to the environment and develop a yellow hue. This degradation can be measured using a colorimeter. It can be expressed in a number of ways, including a value on a yellow index.
About the authors
Kristie DeJesus is a product-development engineer for the Product Identification Business Team at FLEXcon, Spencer, MA. She works closely with customers, sales representatives, and manufacturing personnel to develop products that meet the needs of the polymeric-coatings marketplace. DeJesus has a bachelor's degree in manufacturing engineering from Boston University and a master's degree in operations and information technology from Worcester Polytechnic Institute.
Keith Condon is a product-development chemist with FLEXcon. He works closely with customers and other members of the Product Development Team to develop products and offer technical support. Condon is a graduate of Virginia's Liberty University, where he received his bachelor's degree in biology with a minor in chemistry.