By: Jim Duff, New Product Development Project Manager, IPS Adhesives
Approximately one-fourth of today’s plastics applications involve polypropylene (PP), a popular choice because of its strength, durability, moisture resistance, and relatively low cost. But with such advantages comes a challenge: PP has low surface energy (LSE), making it difficult to use adhesives for bonding, meaning designers often opt for more costly and less effective welding or mechanical fastening alternatives. Now, however, advances in adhesive chemistry have spawned a new generation of adhesives designed specifically for bonding LSE materials like PP, giving design engineers new options for adding value to their product designs and making life easier for assemblers and fabrication workers across many industries.
Why PP is difficult to bond
Polypropylene has a low surface energy, which means its molecules do not readily attract or adhere to other substances. This property is due to its non-polar molecular structure, composed mostly of carbon and hydrogen atoms without polar functional groups like hydroxyl (OH), a structure which offers fewer sites for the hydrogen binding that is necessary for adhesion to many surfaces.
PP is chemically inert and resistant to many solvents, acids, and bases. This resistance further reduces its reactivity with adhesives and coatings, making it challenging to establish strong bonds. Polypropylene often has a semi-crystalline structure, where polymer chains are arranged in ordered crystalline regions. This structure reduces the surface area available for bonding, as compared to materials with an amorphous structure.
Bonding options
Because of its low surface energy, anyone wishing to fasten PP to other substrates has a limited number of options. When bonding PP to itself, designers might increase the surface energy through hot welding techniques or plasma treatments. Increasing the surface temperature of PP may also enable it to bond to more standard adhesives that do bond to metals. Such approaches, however, can be complex and add a step to the process.
Fastening PP to metals such as stainless steel, aluminum, or other substances often involves mechanical methods such as rivets, bolts, clips, and tapes. These methods can be effective but also add complexity, weight, and cost, to the process.
Today, thanks to advances in adhesive chemistry, product designers can specify adhesives specifically formulated to bond PP to metals, other plastics, and many other surfaces. While each adhesive manufacturer has its own proprietary range, many offer formulas designed to bond to low surface energy materials. Specific formulations vary in installation practices and performance, so a comprehensive evaluation of product claims, features, and benefits is essential before specification.
IPS Adhesives SG400LSE: The advanced adhesive technology
A prime example of a modern adhesive tailored for low surface energy (LSE) materials is the IPS Adhesives SG400LSE. This adhesive has been engineered specifically to bond difficult-to-adhere-to plastics such as polypropylene (PP), which traditionally posed challenges due to its non-polar, chemically inert nature.
The adhesive offers several key advantages. First, it provides exceptional bond strength. Despite PP’s low surface energy, it creates a strong, durable bond without requiring extensive surface preparation or treatments like plasma or chemical primers, which saves both time and cost in the assembly process.
Another significant benefit is its curing time. This adhesive sets within six hours and cures fully within 24 hours, allowing for efficient assembly processes that are well-suited to high-speed manufacturing environments.
Additionally, IPS Adhesives SG400LSE is formulated to resist sagging, even when applied on vertical or overhead surfaces. This low sag property ensures that the adhesive stays in place during the curing process, leading to more precise and aesthetically pleasing results.
Selecting an LSE adhesive
LSE capable adhesives differ regarding the materials they claim to bond, surface treatment requirements, tests completed and published, environmental resistance characteristics, curing and set times, sag performance, and the availability of user testimonies. Here are some things to watch for:
- Surface compatibility. Read the fine print. While many packages say they bond plastic, some work well with certain plastics but not others due to differences in surface characteristics. Surface preparation requirements may also be stated If PP is your immediate need, be sure it is specifically mentioned.
- Implementation. Review and follow vendor guidelines closely. LSE bonding can be tricky and there is little tolerance for variation from the established procedure.
- The curing and setting time. How long does it take the bond to set initially and finally cure? Most LSE adhesives will set within about six hours and cure fully in about 24 hours. Be wary of any process that claims faster or significantly longer duration.
- Test results. LSE adhesives are still relatively new on the market, so many companies may still be testing their adhesives on various materials, but some results are emerging. Most vendors will publish what they have tested and will provide samples so you can do your own testing.
- Low Sag. Low sag is the ability of the adhesive to resist flowing or slumping when it is applied to a substrate vertically or overhead. This property is valuable particularly when bonding two surfaces that are not in a horizontal position. A low sag adhesive would not drip or run, enabling more precise product application. More precision improves bonding by maintaining a consistent bonding surface, improving reliability.  It also saves money by reducing waste and improves overall productivity by ensuring consistent application of the adhesive.Â
- Bond strength: After allowing the adhesive to cure fully according to the manufacturer’s instructions, test the bond strength. Try to peel or stress the bond to see if it holds up to normal use conditions. It is important to do this 24 hours, and then maybe each month. If you get past three months, you can be confident that you have a permanent bond. (Note that it is often the case that the substrate will fail before the adhesive.)
- Environmental stability: Review environmental factors such as temperature variations, moisture, UV exposure, and chemicals, depending on where the plastic will be used.
- Support:Â Vendors are anxious to expand applications for their products, so most will be more than happy to work closely with users to understand how their adhesives can achieve new bonds.
Applications
Polypropylene is used in transportation, consumer electronics, consumer goods, packaging, outdoor furniture, signage, industrial prototyping, and a wealth of other applications.
Transportation
The transportation industry, especially the automotive industry, is among the largest PP consumers because of its lightweight and easy moldability. To increase surface energy, many automotive designers rely on plasma surface treatment, which can be extremely expensive, just to have the process running. Removing this step from the process can save thousands and shorten product delivery times. Others join surfaces with screws, bolts, and clips, for their permanent hold and ease of implementation. Another option is to design clip points into plastic components, so they just clip together. Choosing a mechanical fastener for a transportation application can lead to a heavier, noisier, and more expensive solution than using an adhesive. The larger the component, like a large truck or train panel, the greater the need to maintain high strength bonding without adding the weight of mechanical fasteners.
Building components
Builders often choose PP for applications like facades and cladding due to its weather resistance and aesthetic appeal. They use several tactics to overcome the LSE of the material, including surface treatments, primers, and mechanical fasteners. Adhesives designed for bonding LSE materials allow these panels to be securely attached without mechanical fasteners, maintaining a sleek, uninterrupted appearance. Low-sag capability can further enhance the aesthetics.
Consumer goods
Designers of various consumer products often start out using mechanical fasteners. Adhesives, however, offer advantages in terms of aesthetic appeal (no visible fasteners), structural integrity, and simplification of assembly processes. For instance, one manufacturer needed a smooth PP surface for a product that required a particular touch and feel but also needed to be glued to an aluminum backing to provide the necessary structural integrity. The ability to bond PP to aluminum was the perfect solution for this application.
Packaging
Many modern packaging materials include combinations of PP and LSE materials (such as certain plastics or coated papers). Having an adhesive that reliably bonds these materials allows for more versatile and innovative packaging designs, for instance, creating packaging that securely holds together different types of materials to improve product protection and presentation.
Outdoor Furniture
Screws, bolts, nuts, and rivets are traditional mechanical fasteners used in outdoor furniture, providing strong, reliable joints, but often requiring periodic maintenance to prevent rust and corrosion. The ability to use high strength adhesives, bonding plastic materials to themselves and metals, especially aluminum and especially for adding decorative components, presents a new opportunity for furniture designers to add value and cut costs.
Signage & display
Many signage and display designers prefer PP signage for its durability, lightweight properties, cost-effectiveness, and ability to resist environmental assaults, but because of the LSE, they had few adhesive options that would deliver its benefits. Signage often involves materials with low surface energy such as polyethylene (PE), polypropylene (PP), and powder-coated metals. LSE adhesives are formulated to adhere well to these substrates, providing reliable bonding where traditional adhesives may fail.
Durability is another important design consideration for signage applications. PP can withstand outdoor environments, including exposure to UV radiation, temperature fluctuations, moisture, and chemicals. This durability ensures that signage remains securely bonded and maintains its integrity over time. Adhesives also often provide a clean, seamless bond without visible gaps or residues, which is crucial for maintaining the professional appearance of signage. These types of aesthetic considerations are particularly important in applications like outdoor displays and architectural signage.
Industrial prototyping
Prototyping is critical to American industry and frequently involves a variety of materials, including low-surface energy plastics like polypropylene (PP), polyethylene (PE), and Teflon (PTFE). LSE adhesives enable designers and engineers to bond these challenging materials effectively, expanding the range of prototype designs and functionalities.
Using adhesives specially formulated to bond to LSE substrates allows for more intricate and innovative prototype designs. Designers can incorporate different materials with varying properties (such as flexibility, durability, or chemical resistance) into prototypes without being limited by bonding constraints. They can also streamline the prototyping process by reducing the need for mechanical fasteners, complex welding processes, or surface treatments that may require additional time and resources. This efficiency can lead to faster turnaround times and reduced prototyping costs.
Storage and containment
PP is commonly used for a wide variety of storage and containment systems, including fuels, chemical storage tanks, fuel tanks, wastewater storage, and hazardous materials. An adhesive that bonds well to PP can help ensure leak-proof joints and seams, enhancing the reliability and safety of chemical storage systems, without mechanical fasteners that could contribute to leaks exposure to open flames. In each of these applications, the ability to bond polypropylene effectively with adhesives contributes to improved durability, safety, and operational efficiency. It enables manufacturers to create robust and reliable containers and tanks that meet stringent performance requirements across diverse industries.
Only the beginning
These examples are just a glimpse of what can be achieved when product design engineers are no longer restricted by the inability to bond polypropylene. This newfound capability not only allows for enhancing durability, strength, and cost-effectiveness in existing PP applications but also opens doors to discovering entirely new applications for polypropylene.
