Summary

Of all occupational accidents, falls from heights are especially serious, not only because of the high compensation costs involved, but also because of the high rate of severe or even fatal injuries associated with them. Quebec occupational safety regulations stipulate that workers must be protected when they are exposed to a risk of falling from a height of three metres or more. If the source of the risks cannot be eliminated or collective means of protection cannot be installed, one protective measure that can be taken against falls from heights is to wear a personal fall arrest system (PFAS) that includes a full body harness, a connecting system consisting of a lanyard and an energy absorber, and an anchorage. To allow the worker vertical mobility, the anchorage is usually a vertical lifeline. These lifelines are made of polymer fibres whose performance can degrade under prolonged exposure to weathering agents such as ultraviolet (UV) radiation and moisture. Despite the crucial role that lifelines play in arresting falls, the way their mechanical strength declines with wear over time is often poorly understood and the deterioration criteria used lack a scientific foundation.

This project concerns the impact of prolonged weathering on the properties of certain vertical lifelines, especially their mechanical strength. Seven types of rope were studied: kernmantle, multiline, polyamide, polyester, polyethylene, polypropylene and Polysteel^TM. The ropes were submitted to accelerated aging in a laboratory environmental chamber and to static natural aging (sun and weather, but without use). Following 6, 12, 18 and 24 months of natural exposure, mechanical and physicochemical characterization tests were performed to track changes in rope properties. The ultimate goal of the project was to build an experimentally validated model (logistic equation) to extrapolate material strength on the basis of length of environmental exposure.

The physicochemical characterization of the ropes, which is required to determine the various aging mechanisms, focused on identifying possible changes in the chemical structure of their polymer fibres, especially with respect to molar mass. To this end, infrared spectroscopy analyses were performed to identify any new absorption bands, which are often related to oxidation reactions occurring as a result of the chain scission  of the polymers, as well as rheology or capillary viscometry testing to estimate the viscosity of the polymer melt or in solution. Viscosity is related to the molar mass of the polymers. Similarly, differential scanning calorimetry analyses were conducted to assess the impact of the aging treatments on the crystallinity of the polymer fibres, in addition to visual inspections and scanning electron microscopy analyses to assess possible changes in fibre morphology as a result of the aging treatments. Mechanical characterization of the ropes was done on the basis of tensile breaking test results, taking the breaking force as the dependent variable, in accordance with the applicable standards and current industry practices.

The breaking tests performed on the lifelines subjected to aging treatments showed that, among the ropes studied, kernmantle exhibited the best behaviour in terms of retention of mechanical strength. The remarkable performance of kernmantle with respect to aging can be attributed to its core-sheath construction. Polypropylene and Polysteel, on the other hand, offered poor performance when exposed to aging treatments. Physicochemical characterization testing of the ropes after aging revealed that the polypropylene and Polysteel models had suffered a generalized oxidation process that, in the case of polypropylene, caused a significant reduction in the molar mass of the polymer fibres. The multiline, polyamide and polyester ropes suffered moderate loss of mechanical strength, whereas aging treatments had virtually no effect on the strength of the polyethylene ropes, despite the fact that their initial mechanical strength did not meet the minimum criteria stipulated in the standards. Polyamide ropes shrank in length, possibly due to a densification of the material. Characterization testing showed that ropes made of polyolefins (polyethylene, polypropylene and their copolymers) are not suitable options as materials for vertical lifelines.

With the exception of kernmantle, all the mechanical strength curves based on time of exposure outdoors were modelled satisfactorily by means of the logistic equation. According to the parameters of the model for each lifeline, the theoretical residual strength of multiline and polyester ropes will not drop below the minimum threshold of 27 kN, even if they are exposed to weather for a very long time (5 years or more), whereas polyamide, polypropylene and Polysteel ropes decline to the minimum threshold of 27 kN after 4 years, 0.5 years and 1 year respectively.