Yes, PVC cement can go bad over time. Typically, its shelf life is about two years if the can is unopened and stored properly in a cool, dry place. Once opened, air can enter the can, causing the solvents essential for creating a strong bond to evaporate, leading to the cement thickening and eventually solidifying. If the PVC cement has become thick, has solid chunks, or has a very strong, acrid smell different from its usual solvent smell, it's a sign that the cement has gone bad. It's crucial to check the cement before use, as using deteriorated cement can result in weak pipe joints, leading to leaks or failures in the plumbing system. Always check the expiration date and store the cement according to the manufacturer’s instructions to maximize its shelf life.

Polypropylene (PP) is a versatile plastic commonly used in a variety of applications, including packaging, textiles, automotive parts, and consumer goods. However, environmental concerns and the push for sustainability are driving the search for alternatives. Notable alternatives include:

1. **Polyethylene (PE)** - Similar to PP in many aspects but with different mechanical and thermal properties, making it suitable for various packaging applications.

2. **Polylactic Acid (PLA)** - A biodegradable polymer made from renewable resources like corn starch, offering a more sustainable option for disposable products.

3. **Polyhydroxyalkanoates (PHA)** - Biodegradable plastics produced by microorganisms, suitable for packaging and agricultural films.

4. **Thermoplastic Starch (TPS)** - Made from natural starch, TPS is biodegradable and used in packaging and agriculture.

Each alternative has its own set of properties and applications, making the choice dependent on specific requirements such as strength, flexibility, biodegradability, and cost.

The coefficient of friction (CoF) between polypropylene and concrete can vary based on factors such as the finish of the concrete surface and the condition (wet or dry) under which the measurement is taken. Generally, polypropylene has a relatively low CoF, indicative of its slick surface characteristics, making it a preferred material for applications requiring minimal friction. For dry conditions, the CoF can generally range from 0.2 to 0.3. It's important to note that surface treatments on concrete or polypropylene can significantly alter this value. In designing applications involving polypropylene sliding over concrete, consideration of these factors is crucial for safety and performance. For instance, adding a texture to the concrete or modifying the polypropylene surface can increase the CoF, providing more grip. Always consider consulting datasheets or conducting empirical tests for the most accurate values relevant to your specific application.