Chapter 10 Characteristics of Plastic Materials

Objectives Advantage of plastics over other manufacturing materials Advantage of thermoplastics Thermosetting plastics

Introduction Plastics are synthetic (engineered or human created) materials that are capable of being formed or molded to product finished products. Plastics are most often made from petroleum base stocks or natural gas. Innovative companies are searching for new ways to make plastics that do not depend on petroleum based products.

Unique Characteristics of Plastics Most plastics weigh about one eighth as much as steel. Plastics are an ideal candidate for product applications where weight affects performance and cost. Saving weight through the use of plastics is critical to the automotive industry. Plastic grocery bags require less energy to produce and transport than paper grocery bags. Plastics are good thermal and electrical insulators making them ideal for cooking utensil handles and housings for portable electric tools.

The Development of Plastics Bakelite (phenol and formaldehyde) was developed in Bakelite was widely used for telephone housings, pan handles, and electrical insulators. Major developments since then include rayon, cellophane, nylon, PVC (poly vinyl chloride), Saran, Teflon, polyethylene, Velcro, and others. The two most common types of plastics (95%) that are recycled are PET (poly ethylene terephthalate) and HDPE (high density poly ethylene). PET is used to make clear blow molded containers such as soft drink bottles. HDPE plastics are used to make more durable bottles for juice, milk, water, laundry, and cleaning products.

Making Industrial Stock Commodity resins are used for molding or fabricating accessories and containers such as garbage bags, toys, and automotive accessories. Engineering resins are used in applications such as football helmets, scientific lab equipment, reheatable food containers and industrial conveyor rollers. Resin as Industrial Stock: Resin is manufactured in pellet, granule, powder, and liquid form. Wood, rubber, and cotton are natural polymers.

Major Classes of Plastics The two major classes of plastics are thermoplastics and thermosetting plastics (thermosets). Thermoplastics can be reheated and reformed repeatedly; thermosets cannot. Thermoplastics are more resistant to cracking and impact damage than thermosets. Thermosets are superior in terms of high temperature strength and chemical stability.

Creep Creep is the slow and continuous increase in length over a period of time when the material is placed under load. Creep increases more rapidly at higher temperatures. Plastic materials must be used conservatively in applications where continued high loadings and temperatures are necessary.

Summary Plastics are most often made from petroleum base stocks or natural gas. Innovative companies are searching for new ways to make plastics that do not depend on petroleum based products. Most plastics weigh about one eighth as much as steel. Saving weight through the use of plastics is critical to the automotive industry. The two most common types of plastics (95%) that are recycled are PET (poly ethylene terephthalate) and HDPE (high density poly ethylene). PET is used to make clear blow molded containers such as soft drink bottles. HDPE plastics are used to make more durable bottles for juice, milk, water, laundry, and cleaning products. Wood, rubber, and cotton are natural polymers. The two major classes of plastics are thermoplastics and thermosetting plastics (thermosets). Thermoplastics can be reheated and reformed repeatedly; thermosets cannot. Creep is the slow and continuous increase in length over a period of time when the material is placed under load. Plastic materials must be used conservatively in applications where continued high loadings and temperatures are necessary.

Home Work 1. What are the 2 most common types of plastics that are recycled? 2. What is the advantage of thermoplastics over thermosets? 3. What is creep?