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When working with automotive pressure-sensitive adhesives (PSAs) — or most any purchase in the supply chain, for that matter — three factors weigh heavily on the buying decision:
Finding the right materials, at the right price point, with a reliable unit volume is a straightforward goal, but it often takes some time. Suppliers can have dizzying catalogs of films and tapes to sift through with nuanced differentiators that make it tough to compare specs tit-for-tat. You may go into the search with a clear idea of the automotive design problem you need to solve but less certainty about the ideal polymer or adhesive formula. A lower price point might not provide much value if the material is poorly matched to the task.
Modern automotive PSAs are versatile, too. Every area of the vehicle will have some specialized options: matte black vinyl paint replacement tapes window sashes and door pillars, for instance. However, as synthetic adhesives have evolved, their material advantages have begun to expand and overlap across multiple use cases and purposes.
Film vs. Foam PSAs
Synthetic films and foams are two of the most common carriers (or cores, or backings) for automotive PSAs. Each style has unique properties that will suit it to certain tasks or substrates, and both have the ability to accommodate single or double-sided adhesion. Acrylic, silicone, rubber, or other adhesive layers will also affect the strengths and value of the PSAs.
Neither film nor foam is specifically “better.” The perfect selection comes down to a harmonious match of carrier, adhesive, substrate, purpose, and anticipated environmental stresses. To settle on the correct PSA for your project, we must start with an examination of the qualities and resistances of each polymer.
Most of the options below can actually vary wildly in forms and properties, depending on the specific formulation. It’s generally possible to find each one in transparent, colored, printed, matte, glossy, or other finishes as necessary. What holds this group together is that they’re straight-edged, thin-gauged, and better suited for smooth substrates than rough or uneven surfaces. Films are also the material of choice when you need transparent adhesives.
Here’s just a sampling of film material options and potential advantages:
- Polyester (PET, polyethylene terephthalate): Mylar is an example of a polyester-based material. These films are light, durable, and optimal for protective shielding or as labels in areas that will experience high temperatures or chemical exposure.
- Acrylic: Exterior automotive films that must endure harsh UV radiation often use this thermoplastic resin, which also boasts excellent clarity and stability.
- Polyethylene: If you need a film that still repels chemicals, corrosives, and moisture, but has more conformability than polyester, polyethylene varieties are your PSA of choice. This is a common material in packaging and temporary protective films.
- Polyimide: This high-performance resin is used for flexible circuit materials and other dynamic roles that take advantage of its high heat resistance as well as electrical and chemical traits.
- PVC (polyvinyl chloride): PVC is known for its ability to resist wear, abrasions, a wide variety of chemicals, and weathering, but it loses strength in high heat applications (unless heat-stabilized).
- Polyurethane: Some “clear bra” paint protection films utilize polyurethane-based formulas for their surface followability, clarity, and toughness in thin-gauge applications.
- Fluoropolymer (PTFE, polytetrafluoroethylene): PTFE enjoys superior flexibility, a low coefficient of friction, and very little stickiness or tackiness on the non-adhesive side. Like most synthetic films, it can also be used to repel chemicals and moisture.
- Polycarbonate: Somewhat less common in automotive PSAs, polycarbonate nonetheless is easily thermoformed for film applications. It has outstanding optical clarity, impact strength, and dimensional stability, die-cuts well, and features excellent ink adhesion (unlike many plastics). This makes it a strong choice for labeling, graphic, and protective uses.
Whereas films are best used to shield surfaces from scratches, rough frictional contact, and corrosive liquids, foams are better at filling gaps, dampening vibrations, and creating flexible bonds, even on irregular, textured, or rough surfaces. In protective roles, foam will not withstand abrasive forces like a film but serves as a shock absorber, diffusing vibrations and impact stresses across the full bonded surface.
There are also two primary styles of foam PSA: open-celled and close-celled. The former will allow air and liquids to pass through, and the latter will not. Closed-cell foam tapes are an excellent option for gasketing and moisture sealing applications.
Here are several of the more common materials for foam automotive PSAs:
- Urethane: As an open-cell foam, urethane will not provide solvent or moisture resistance. However, it excels as thermal insulation or a sound and vibration dampener. Note that urethanes do not perform well in exterior applications where weather and UV radiation will degrade them over time.
- Polyethylene: This material produces an economical closed-cell foam tape that’s often selected for sealing applications. It is also a cost-efficient choice for insulating or cushioning.
- Vinyl: Vinyl foams are closed-cell, highly flexible, and resistant to both heat and UV exposure. Automakers use them as sealing PSAs to block out dust, moisture, and light.
- Acrylic: Exterior trims are often adhered with acrylic tapes, which have outstanding longevity and durability in extreme environments as well as strong adhesion.
Selection by Use Case
The purpose of the PSA is likely the first filter in your decision-making process. In general, films offer the best performance when you need an adhesive to be thin, clear, printed, or to act as a shield against liquid or abrasion. Foam PSAs are at their best when their flexibility comes into play, as with gap filling, shock absorption, sound dampening, or stretchable bonding between substrates with dissimilar thermal expansion properties.
For example, if you need a temporary protective solution — such as in packaging or shipping of auto parts — the economical price point and abrasion resistance of a polyethylene film, when paired with a low-tack adhesive layer that will leave no residue, makes for an easy choice.
However, if you’re trying to bond two rough or textured surfaces, a double-sided film tape will not perform nearly as well as a gap-filling foam tape. Flexible foams will hug uneven surfaces as the cells compress around and conform to irregularities, allowing better adhesive wet-out and bond strength.
Here is some brief advice on how to select the right automotive PSA for several common design problems and use cases:
- High Temperatures: Polyester, vinyl, and polyimide films are highly thermal-resistant and often used in the extreme area of the engine compartment. PVC, while less resistant to heat, can be stabilized for high temperatures with a variety of additives.
- Cabin Noise: Vehicle and component rigidity has a massive impact on cabin noise. Double-sided automotive film PSAs can boost rigidity and reduce noise from squeaks or rattles by providing a more consistent, full-surface bond than mechanical fasteners. However, anti-vibration foam tapes are even better when bond strength is less important than diffusing and silencing troublesome vibrations.
- Bonding Dissimilar Substrates: Both foams and films can be fitted with a unique adhesive on each side (to suit the substrate), but foams often perform better here. This is because plastic and metal surfaces generally respond differently to heat. Even two plastics, like polyvinyl chloride (PVC) and glass-reinforced polycarbonate, may have very different thermal expansion ratios. The viscoelastic properties of foams allow the bond to flex as the temperature fluctuates. Acrylic foam tapes are a common choice for bonding plastic emblems and trim onto exterior metal panels.
- Chemical Resistance: Most synthetic polymers enjoy high chemical resistance. Some of the more notable materials chosen to resist oil, gas, coolant, road salt, grime, and more include polyester, PVC, PTFE, vinyl, and acrylic. All can be suited to either film or foam formats, depending on your purpose.
- Sealing and Insulation: Weatherstripping will often use closed-cell acrylic or polyethylene foam, which won’t permit the pass-through of moisture and will fill gaps with a tight seal. Urethane foams — which are open-cell — are not useful for sealing out moisture but excellent for dampening sound. Vinyl films are excellent blackout tapes where you need to cover production holes and prevent light from penetrating.
- The Substrate Outgasses: Many injection molded thermoplastics will outgas over time, and they can create bubbles or blisters under film PSAs as a result. An open-celled polymer, while technically a foam, can be designed for use as a fine-celled, air-permeable “film” that is indistinguishable to the eye from other film PSAs. These blister-free films allow outgasses to pass through the cellular structure without creating bubbles and are often used for labels and bumper stripes.
This list provides food for thought, but there are many, many more potential considerations when selecting the right film or foam PSA for automotive purposes. It’s always best to consult with your supplier for guidance before requesting a quote on a particular PSA. They may have options you haven’t considered in their portfolio that could maximize the value of your purchase. Suppliers are highly motivated to earn your ongoing business, and they will be happy to work with you to find the best possible solution.
Image Credit: Nomad_Soul