In formulating adhesives, one often is faced with the choice of using a homopolymer or copolymer as the base polymer. There are significant differences between these two types of polymers, and the correct choice will be an important factor in the application and performance properties of the final product.
To understand the difference in properties, it is necessary first to appreciate the structural differences in these materials.
A homopolymer is formed from only one type of monomer. When a single type of monomer is polymerized to form a polymer, it is known as a homopolymer. In other words, there is one repeating unit.
A copolymer is formed from two different types of monomers. These monomers join in various fashions to form the polymer.
The ways in which a copolymer forms a polymer determine its classification. If the two monomers are arranged in an alternating way, it is known as an alternating copolymer (e.g., if the two monomers are designated A and B, the alternating copolymers can be illustrated as ...ABABABABAB...). The monomers can also be arranged in a random fashion as ...AAABABBBAABBAAAB...these are called random copolymers. Sometimes each monomer will join with like monomers to form blocks of monomers such as ...AAAAABBBBBAAAA... these are called block copolymers. Graft copolymers are branched copolymers in which the side chains are structurally distinct from the main chain. These are the most common types of copolymers. There are several others that can be classified by the geometric or statistical relationship in which the polymer forms.
It should be emphasized that a copolymer is not a mixture of polymer, but a chemical compound that may have entirely different properties from those of either of the pure monomers that make up the copolymer. The copolymer of styrene and butadiene (styrene butadiene), for example, is different from both polystyrene and polybutadiene. It is a synthetic rubber whose hardness and wear qualities depend on the relative portions of styrene and butadiene in it.
Because two, three, four, or even more monomers can be copolymerized, it is possible to synthesize a great many polymers from a few monomeric materials. By varying the kind and relative amounts of the monomers, polymers with a wide range of mechanical and chemical properties can be produced. As a result, copolymers have become widely used in certain types of adhesive systems. Commercially relevant copolymer base polymers that are used in adhesive formulations are listed below:
Vinyl acetate copolymerized with ethylene, acrylate, maleate, and other monomers
Styrene block copolymers such as styrene-butadiene-styrene (SBS), styrene-isoprene-styrene (SIS), styrene-ethylene-butadiene-styrene (SEBS), and styrene-ethylene-propylene-styrene (SEPS)
Acrylic copolymers based on monomers of methyl acrylate, ethyl acrylate, n-butyl acrylate 2-ethylhexyl acrylate, etc.
Copolymerization is used to modify the properties of polymers for specific adhesive applications. For example, it can be used to reduce crystallinity, modify glass transition temperature, or to improve solubility. It is also a way of improving mechanical properties such as toughness, impact resistance, and peel strength by providing greater elongation without sacrifice of strength.
The property differences of homopolymers and copolymers are listed in the Table below. Many homopolymers are not suitable for adhesives or sealants because of their rigidity and hardness. As a result, copolymers are commonly used.
- High Tg
- Hard and brittle
- Limited adhesion (bond strength to non-porous surfaces)
- High cohesion (resistance to tensile and shear stress)
- Limited use as a pressure sensitive adhesive (low peel strength and tack)
- Limited use at low temperatures
- Generally requires external plasticizers
- Lower Tg
- Soft and flexible
- Improved elongation (high peel strength, toughness, and impact resistance)
- Broad adhesion to a variety of substrates
- Internally plasticized
- Internal modification possible via concentration and order of differing monomer groups
- Limited use at high temperatures (depending on the Tg)
Table 1: Properties of Homopolymer and Copolymer Emulsions
Important to the Adhesive or Sealant Formulator
The main advantage of the copolymerization approach is that the modifying additive is "internal' to the molecule (i.e., actually part of the molecule rather than a separate ingredient that can migrate out of the formulation). An excellent example of this is evident in wood adhesive made from polyvinyl acetate. If the homopolymer polyvinyl acetate is used, the resulting adhesive is very stiff and rigid. Although this adhesive has a high shear strength to wood, it does not have impact strength, movement capability, or a high degree of peel strength. These properties, however, can be achieved with the addition of an external plasticizer. However, the use of certain plasticizers is not in favor due to their migration tendency as well as toxicity and environmental considerations. Similar elastic properties can be achieved by copolymerizing the polyvinyl acetate monomer with ethylene to form a vinyl acetate ethylene molecule. When used as a base polymer this resin does not require modification with external additives.
The major advantage of copolymers is that the modifying monomer is "locked' into the molecular structure via chemical bonds. This prevents migration of the additive out of the polymer as may be the case for external plasticizers. The major disadvantages of using copolymers for plasticization are that:
- Internal plasticization must be done at the polymerization stage and, thus, a base polymer is directly manufactured for a specific application (with external plasticizers nearly infinite modifications are possible with a single base polymer).
- Internal plasticization is generally more costly than external plasticization.
- The breadth of property modification is limited due to the chemistry required, and only certain polymers are capable of internal plasticization.
Block copolymers provide another interesting example of the use of copolymers in the adhesives industry. Block copolymers such as styrene-butadiene-styrene (SBS), can "microphase separate' to form nanostructures. The phase separation occurs because of incompatibility between the blocks. This leads to greater toughness, elongation, and strength. The end-blocks of the copolymer (styrene) provide a higher melting temperature than the butadiene mid-blocks and, thereby, the copolymer is physically crosslinked to provide reasonably good heat and creep resistance for a thermoplastic.
Acrylics emulsion adhesives represent yet another way in which copolymers are employed. The use of higher Tg monomers will raise surface hardness, modulus, and water resistance. Certain monomers possess adhesion-promoting and other valuable properties. Improvements to adhesion can be achieved due to the use of more polar monomers. Acidic monomers also provide stabilization during and after polymerization since they are anionic in nature. They also provide viscosity increase if the polymer is hydrophilic. In this way the emulsion becomes self-thickening and no additional thickeners are required in the formulation.