Plastic has become an ubiquitous material in our daily lives. Its versatility extends beyond products made solely from plastic, as it is often used to coat metal products to enhance their aesthetics, durability, and performance. Plastic adhesives play a crucial role in various manufacturing industries, including: Transportation, Medical Devices, Electronics, Consumer Electronics, Energy, Sporting Goods,…

It cannot be denied that plastic materials offer valuable advantages. However, we also need to acknowledge the challenges they present during the design and selection process for bonding and assembly materials. With the variety of glue types on the market and the diverse range of plastic surfaces, how can we choose the right glue for the right plastic and still achieve optimal results? This article will answer why plastic is difficult to bond, how to design plastic bonds, and suggest some highly effective plastic adhesives.

Why is Plastic Difficult to Bond?

Plastics like polyethylene, polypropylene, and polytetrafluoroethylene (PTFE) are among the most challenging to bond due to their low surface energy, ranging from 20 to 40 dynes. Notably, polyolefins and Teflon are two of the most widely used plastics with superior properties and diverse applications. They have unique polymers with distinct characteristics and applications in industrial manufacturing.

PTFE, or polytetrafluoroethylene, is a polymer known as a perfluorocarbon resin. In perfluorocarbon resins, all carbon atoms in the main chain of the polymer chain are completely linked to fluorine atoms. The carbon-fluorine bond (C-F bond) is so strong that neither oxygen nor ultraviolet radiation has enough energy to break it. The strength of this bond is the source of its chemical inertness and good aging properties. The distribution of fluorine atoms around the carbon polymer frame balances the negative and positive charges, making PTFE non-polar. Non-polar materials do not attract polar substances like water. This fully fluorinated polymer has a low surface energy, making it impossible to be wetted by water. PTFE has no known solvents. 

 

Additionally, Teflon also exhibits excellent chemical resistance and high thermal stability, making it an ideal material for various applications such as fluid conduits, non-stick pans, and medical applications. However, Teflon’s non-stick properties also make it difficult to bond with other materials, requiring special surface preparation methods and adhesives to ensure good adhesion.

Read more: Mastering Adhesive Solutions for Tricky Surfaces: Polyolefin and Teflon

In medical device assembly, materials like nylon, COC/COP (cyclic olefin copolymer/polymer), and PEBA (polyether block amide) are preferred due to their superior properties that meet the stringent requirements of the medical industry. However, their chemical structures give them low surface energy, posing challenges when bonding them together. Refer to the article How to bond Hard-to-bond Substrates (Nylon, COC/COP and PEBA) in Medical Devices Assembly?  to learn about medical-grade plastic adhesives and required certifications.

Besides common plastics, new composite materials also present challenges to manufacturers due to the varying surface energy and wettability among the different plastics incorporated in the composite. Prostech has suggested some adhesives for bonding composite plastic materials in this article, click here to read the full article.

Contaminants like dust, oil, fingerprints, etc., left behind from the previous cutting and grinding processes are also issues to consider. If manufacturers do not remove these contaminants before bonding, the adhesive will only bond to the contaminant layer rather than the actual plastic surface.

 

In summary, the reasons why plastic surfaces are difficult to bond are due to the low surface energy of plastics and the presence of contaminants like dust and oil on the surface that have not been cleaned.

To address these issues, before bonding plastics, we need to pre-treat the plastic surface or clean the plastic surface before bonding. Read Prostech’s article on surface treatment methods here.

PLASMA: Surface treatment, increases, decreases surface energy using Plasma, no chemicals.

Chemical surface cleaning: Use solvents to thoroughly clean the surface, considering environmental, fire safety, and health factors when choosing the appropriate solvent.

Contact Prostech to choose a surface treatment solution for difficult-to-bond plastics for your product.

 

Effective Plastic Adhesive Selection Steps

1. Identify the Plastic Surface to be Bonded

• Is the actual surface to be bonded plastic?
  
  Many manufacturers apply powder coating to the plastic surfaces of their devices. In this case, the powder-coated surface is the direct contact surface with the adhesive, not the plastic material itself. For powder coatings, we need to pre-treat them before bonding.

• What type of plastic is the surface to be bonded? Each type of plastic has different properties, so identifying the specific type of plastic will help you choose the right adhesive. What type of plastic is your surface from among the common plastics (PET, HDPE, PVC, LDPE, PP, ABS, Polyolefin, Teflon, etc.)? If it is not one of these, what type is it? Identify it clearly to avoid wasting time searching for the right plastic adhesive.

2. Plastic Surface Treatment Step

As mentioned above, in the processes before the plastic parts reach the assembly step, they have been machined with many solvents and other chemicals such as oil, grease, mold release agents, and dirt that remain on the surface. This reduces the contact area between the surface and the adhesive, leading to weak adhesion.

On the other hand, after using solvents for cleaning, the surface energy does not increase much. Therefore, manufacturers should use more advanced surface treatment technologies such as plasma/corona machines.

However, in many cases, if the plastic surface does not have too many of these problems, simply choosing the right adhesive can provide a strong bond.

3. Designing Plastic Joint Bonds

The force impacting the plastic joint is an important factor that needs to be considered when designing the joint. Engineers need a strong foundation of knowledge about distributing force between the two surfaces in order to design the strongest possible joint.

There are 5 forces that the joint must endure when two surfaces are bonded together with glue/tape:

• Shear Strength (Strength against cutting)
• Compression Strength (Strength against compression)
• Tension Strength (Strength against pulling)
• Cleavage Strength (Strength against splitting)
• Peel Strength (Strength against peeling)

Most adhesives have Shear Strength, Compression Strength, and Tension Strength, but their effectiveness will gradually decrease when enduring Cleavage Strength and Peel Strength over time.

For the most durable joint design, the force distribution must be even across the entire bonded surface.

Prostech has many years of experience in designing joints suitable for various materials and surfaces, especially plastic surfaces. Contact us for consultation.

4. Choosing the Right Plastic Bonding Adhesive

When choosing an adhesive, it’s important to understand the characteristics and strengths of each type. The properties of the adhesive can directly affect production time, resistance to temperature, humidity, chemicals, and external forces on the joint.

For most plastic bonding applications, CA glues, UV adhesives, MMA adhesives, and some other high-strength adhesives can all be used. However, with so many options, it can be difficult to choose the right one.

a. CA Adhesives (Cyanoacrylate Adhesives) for Plastic Bonding

Cyanoacrylate adhesives, also known as super glues, are one-component adhesives that offer fast drying and good adhesion to plastic surfaces.

PROSTECH’s CA adhesives are suitable for most plastic surfaces made from ABS, PMMA (acrylic), Nylon, Phenolic, Polyamide, Polycarbonate, and PVC (both rigid and flexible). For fast-curing CA adhesives, the bond can be very strong on polyethylene and polypropylene when used with POP primer.

Some outstanding features of Prostech’s CA plastic adhesives:

• Good price
• Easy to use
• Little to no white fumes
• Dries fast, within seconds
• Good resistance to high temperatures and chemicals
• Safe for users
• They have certifications including ISO 9001, ISO 14001, UL, NSF International, REACH, RoHS, etc.

See the applications of CA adhesives in various industries:

Catheter Assembly: Choosing the Right Medical Adhesive Grade

 

View detailed product information:

• Permabond 825 plastics bonding adhesive
• GLUDITEC CA51D-401 plastics bonding adhesive
• Loctite 406 plastics bonding adhesive
• HB Fuller M5 100 plastics bonding adhesive
• 3M CA100 plastics bonding adhesive

b. UV Adhesives for Plastic Bonding

Most of PROSTECH’s UV adhesives for plastic bonding work well on ABS, Nylon, Phenolic, Polyamide, Polycarbonate, and PVC (both rigid and flexible) surfaces. Some special UV adhesive codes are specifically designed for use on acrylics.

Outstanding features of Prostech’s UV plastic adhesives:

• Fast drying: UV adhesives can dry in seconds when exposed to UV light, increasing production speed.
• Low or no VOC solvents
• On-demand drying: The adhesive dries when exposed to UV light, so manufacturers don’t need to worry about time, temperature, or the environment like other adhesives.
• High adhesion: UV adhesives have good adhesion to a variety of materials, including plastics, metals, glass, and ceramics.
• Resistant to temperature, chemicals and UV rays: UV adhesives are resistant to high temperatures, chemicals and UV rays, making them suitable for applications requiring high durability or outdoor use.
• Easy to use: UV adhesives are typically used in liquid or gel form and are easy to apply to the bonding surface.

To use UV adhesives effectively, a UV light source is required to activate the polymerization process. Typically, devices such as UV lamps or UV bulbs are used to provide the necessary UV light source. Prostech offers a comprehensive adhesive solution, including: Spot UV curing machines, beam UV curing machines, conveyor belt integrated curing machines.

View detailed product information:

• MXBON 41132M plastics bonding adhesive

• PERMABOND UV643 plastics bonding adhesive

• Araldite 2018-1 plastics bonding adhesive

Read more:

Applications of UV Adhesive in Electronics Industry

What are the advantages of using UV Adhesive?

UV Adhesive: A guide to overcome all the challenges for optimizing its benefits

c, Single-Component Epoxy Adhesive

This type of adhesive dries when heated at high temperatures. Therefore, it is not suitable for plastics that melt at temperatures lower than the curing temperature required for the adhesive to dry. This type of adhesive is typically used with high-temperature resistant plastics or synthetic resins such as PEEK or PBT.

d. Two-Component Epoxy Adhesives for Plastic Bonding

Two-component epoxy adhesives are commonly used for bonding specific types of plastics. PROSTECH’s specialty epoxy product line offers the adhesive strength required for applications demanding high durability and external impact resistance. The re-structurable epoxy line is also utilized in applications requiring both adhesive strength and higher elasticity compared to traditional epoxy types.

 

Characteristics of Prostech’s Two-Component Epoxy Adhesives for Plastic Bonding:

• Extremely high adhesion, withstands heavy loads
• Resistant to high temperatures (from -60°C to +204°C)
• Resistant to chemicals and solvents
• Suitable for bonding metals, plastics, ceramics, and wood
• Waterproof

View detailed product information:

Araldite 2012 plastics bonding adhesive

e. MMA Structural Adhesives / Acrylic Structural Adhesives

Acrylic adhesives are a versatile type of adhesive that can effectively bond a wide range of plastic surfaces. They can be applied in various ways, including applying two components in parallel on the same surface or mixing the components before application. MMAs (adhesives with a methyl methacrylate base) are a particularly effective choice for plastic bonding applications, offering exceptional adhesion strength.

Key characteristics of MMA structural adhesives/acrylic structural adhesives:

• Strong adhesion: These adhesives form a robust bond between plastic surfaces, ensuring a secure and durable connection.

• Versatility: They can adhere to a diverse range of plastics, including ABS, PVC, polycarbonate, and acrylic.

• Fast curing: Acrylic adhesives cure quickly at room temperature, significantly reducing bonding time.

• Good chemical resistance: They exhibit resistance to chemicals, solvents, and moisture, making them suitable for use in harsh environments.

View detailed product information:

• LOCTITE® AA H3151 plastics bonding adhesive
• 3M™ Scotch-Weld™ DP8910NS plastics bonding adhesive
• Devcon Composite Welder FS  plastics bonding adhesive
• HARDLOC G-53-03 – 2K  plastics bonding adhesive
• Plexus® MA1025 plastics bonding adhesive
• Bostik SAF Ultimate M25  plastics bonding adhesive
• PERMABOND® TA4631 plastics bonding adhesive

In this article, we have explored the reasons why plastic surfaces are difficult to bond, the process of designing plastic joints, common types of plastic adhesives, and their effectiveness. To choose the most suitable adhesive for your application, please contact Prostech for consultation and receive free samples.