Material Properties and Behavior
Most construction polymers have thermoset character. Exceptions are very flexible epoxies, some polyurethanes, polyureas, hybrids of these materials and some latex polymers. They are thermoplastic or nearly so.
Thermoset polymers products are made up of highly cross-linked molecular elements, maintain their shape when heated, but do not melt. Above about 240 to 300° F they decompose. Thermoset polymers are rigid (high modulus of elasticity), somewhat brittle at low temperature and have limited flexibility at higher temperatures. The mid-point of the temperature range at which the transition from rigid to flexible takes place is called the glass transition temperature.
The Heat Deflection Temperature (HDT) is a simple indicator of the glass transition temperature. Most construction polymers have a HDT in the range between 65 and 120°F. At temperatures in and above the glass transition region, construction polymers subjected to high loads will deform which leads to loss of bond and ultimate failure of the application.
The fact that many construction polymers are used in the glass transition temperature range should always be considered when recommending a material. Know the HDT of the product you want to use. Many failures can be traced to a disregard of the material behavior changes that occur in this temperature region.
Significantly below the HDT (~20°F) thermoset polymers can withstand substantial loads without deformation (creep) which is the basis for their use in construction (crack repair, bolt and dowel grouting, bonding applications).
Except for truly elastomeric products all construction polymers have more or less pronounced visco-elastic character. When stressed they do not entirely respond in the manner an elastic body(spring) would and they don’t flow in the manner of a liquid (response like a shock absorber to a bump).
The response of a construction polymer to a stress (load) is partly elastic and partly of viscous nature. The prevalence of each of the components in response to a stress (load) depends on the composition of the polymer, the stress level, the stress exposure duration, and temperature. Short term loads generate a quick elastic response. The response to long term loads is seen in creep which is a form of viscous flow.
As a consequence of their visco-elastic nature, construction polymers have time and temperature dependent properties. Typical rigid stress-strain behavior at low temperature changes to elastomeric behavior at high temperature, while increased in strain rate at a given temperature produce a increasingly rigid response. All visco-elastic construction polymers have temperature ranges where they are rigid or flexible. The glass transition temperature separates the two ranges.
In structural applications, it is important that the construction polymer does not creep under sustained load. The tendency to creep depends on the load level and the prevailing temperature. Each material has an envelope where creep does not produce failure at a given temperature and load level.
Construction polymers used in “load bearing” applications should have a high HDT. When such polymers are subjected to permanent loads, particularly when these loads have a high shear component, the surrounding structure element should always remain at least 20° F below the HDT. In critical applications and as a safety measure in case of a fire thermal protection should be provided. In recognition of the material performance needs in load bearing applications, ChemCo products were deliberately designed to have higher HDTs than are typically offered by the industry.
Thermoplastic polymers melt when heated and return to their original state when cooled again, unless they were heated to a point above their decomposition temperature. Their glass transition temperature is very low (< -40° F). Thermoplastic and slightly cross-linked thermoplastics have largely predictable properties in normal construction polymer use temperatures because they are used above their glass transition temperature. Exceptions are polymers with crystalline segments within their molecular structure that melt in this region. They are more or less elastic (low modulus of elasticity) and get largely employed for this property (elastomer membranes, joint materials, roofing compositions, seals).