Bonding of plastics

Bonding plastics is a very extensive topic. In this article, we will try to describe in detail all aspects of joining popular plastics.

What are plastics?

Plastic materials are materials consisting of synthetic polymers (man-made and not found in nature) or modified natural polymers and modifying additives such as powder or fibrous fillers, thermal stabilizers, UV stabilizers, flame retardants, antistatic agents, foaming agents, dyes. These polymers most often resemble chains in shape. Polymer chains consist of monomers, i.e. individual molecules, between which there are chemical bonds that form the chains discussed above. Between the polymer chains themselves there are chemical and physical connections as well as mechanical weaves, depending on the type of plastic.

Breakdown of plastics

Plastic materials differ in their reaction to temperature, various solvents and mechanical loads. The following types of plastics can be distinguished:
- thermoplastics
- duromers
- elastomers
- thermoplastic elastomers

Thermoplastics

Thermoplastics / Therplastics – plastics that become liquid and sticky at certain temperatures and pressures. Thermoplastics can be shaped by extrusion and injection at elevated temperatures, followed by rapid cooling to the operating temperature. Thermoplastics can be processed many times, unlike duroplastics, but after each processing, their functional and mechanical properties usually deteriorate due to depolymerization and degradation of the polymers or resins that make them. This negative phenomenon can most often be observed in HOT MELT hotmelt adhesives, e.g. based on EVA, where the tank is often heavily soiled with various scales from burnt resin during use.

Thermoplastics have a structure of interwoven polymers that physically interact with each other. In addition, there are numerous mechanical weaves. Their characteristic properties are fusibility and weldability, and if they are subjected to certain solvents, they swell or dissolve.
The most commonly used thermoplastics include:

• polyethylene (PE)
• polypropylene(PP)
• polystyrene (PS)
• polyvinyl chloride (PVC)
• poly(methylene oxide)(PMO)
• poly(ethylene terephthalate)(PET)
• polymethyl methacrylate (PMMA)
• polyamide (PA)

Duromers

Duromers - polymeric plastics with unique features of metal or ceramics - hard, inflexible, with high mechanical resistance to stretching and compression. Usually non-melting, non-weldable and temperature resistant, often brittle. Duromers are characterized by a very high cross-linking. There are chemical bonds between the polymer bonds. Accordingly, they are insoluble and hardly swell. The best-known duromers include:

• cross-linked:
• bakelite and other phenolics
• cross-linked polyester resins
• cross-linked epoxy resins
• not crosslinked
• poly(ethylene oxide)
• polyacetylene
• poly(vinyl acetate)
• polyacrylonitrile
• certain types of polyamides, e.g. Kevlar
• polybenzimidazole

Elastomers

Elastomers - are polymeric plastics or natural materials that are capable of reversible deformation under the influence of mechanical forces, while maintaining the continuity of their structure. Elastomers are a broader group of materials than rubbers, which are just one class of elastomers. Between the polymer chains in elastomers there are chemical and physical bonds as well as mechanical weaves. These plastics are non-weldable, non-melting and insoluble, but tend to swell in certain solvents. They are characterized by a high tendency to creep.

TPE thermoplastic elastomers - these are multi-phase polymeric materials that behave "rubber-elastic" like elastomers in the temperature range of use, but above this temperature, unlike elastomers, they fall into the flow range. The so-called soft phase has the properties needed to impart rubber elasticity. Therefore, they can also be called TPE thermoplastics. Unlike elastomers, which can no longer be converted at elevated temperatures after cross-linking, thermoplastic elastomers are processed like thermoplastics. Most TPEs are weldable and fusible. They also have swelling and dissolving propertiesin certain solvents.

Some types of TPE:
- EPDM/PE - ethylene-propylene-dimer/polyethylene
- EPDM/PP - ethylene-propylene-dimer/polypropylene
- EVA - ethylene-vinyl acetate
- NR/PP – natural rubber/polypropylene
- SB/S – block polymer styrene/butadiene/styrene
- S/EB/S – styrene/ethylene-butadiene/styrene
- TPE-A - thermoplastic polyamides (TPA)
- TPE-S - thermoplastic styrene copolymers (TPS)
- TPE-U - thermoplastic polyurethanes (TPU)

Why is it important to identify the plastic before gluing?

Adhesives, as I mentioned in previous publications, are non-metallic materials that harden to plastics. If we know the basic properties of the material we want to glue, plus we know the conditions they will face (see the article on designing adhesive joints), we can select the appropriate adhesive that, having the desired properties, will allow us to create a durable adhesive joint.

Adhesives can be classified in terms of belonging to particular groups of plastics as follows:

• Adhesives with the characteristics of thermoplastics:
  - Hot Melt adhesives but without reactive PU
  - water dispersion
  - solvent adhesives
  - instant adhesives (CA)
  - UV adhesives
  - rarely MMA
  - plastisols
• Adhesives with characteristics of duromeric materials:
  - EP 1K/2K adhesives
  - PU 2K / reactive PU (HOT MELT) adhesives
  - anaerobic adhesives
  - very rarely UV adhesives
  - MMA adhesives
  - phenol-formaldehyde resins
• Adhesives with elastomeric characteristics:
  - 1K PU / 2K PU / reactive PU (HOT MELT)
  - 1K Silicone / 2K Silicone
  - 1K MS Polymer / 2K MS Polymers