Plastic windshields in motor vehicles
The various windows used in motor vehicles must be certified as part of the homologation process. The requirements of UN Regulation 43 apply to the certification and govern the so-called safety glazing materials and their installation. This regulation also applies to almost all glass and plastic windows. Exceptions apply to plastic windshields since to date they have not been certifiable in Europe. However since 1988 a special national regulation permits plastic windshields for special vehicles designed for a maximum speed of 40 km/h. In 2001 a special approval was issued by the German Federal Ministry of the Interior to extend the use of plastic windshields in all German police vehicles up to the maximum speed of the vehicle.
The experience gathered to date with plastic windshields demonstrates the wide range of advantages that they offer compared to glass windshields, which are used as standard at the current time. Plastic windshields are lighter and allow a wide variety of shapes to be manufactured. They are drastically less susceptible to fracture than glass windshields, which is why they are particularly used in forestry and agricultural vehicles. The subjects of durability, scratch resistance and resistance to ultraviolet radiation must be given special consideration for the use of plastic windshields. The various types of plastic and different surface coatings require trials and field tests.
Plastic windshields must satisfy the pedestrian safety requirements if they wish to achieve general homologation. In addition, a plastic windshield must not represent an obstacle to rescue workers in the event of an accident. The rescue workers must be able to gain access to the accident vehicle regardless of whether the vehicle has a glass or plastic windshield.
In April 2010, an informal working party led by Germany was formed by the United Nations Economic Commission for Europe (UNECE). The objective was to prepare test methods for the homologation of plastic windshields. The work of the informal working party was completed in 2014. In October 2014, an amendment proposal for UN Regulation 43 was approved in the UNECE Working Party on General Safety Provisions (GRSG).
This amendment proposal defines the test requirements for plastic windshields and contains the following criteria: Evaluation of the light permeability, fire properties, chemical resistance, weather and moisture resistance, and temperature, ultraviolet and temperature cycle resistance. In addition, the following tests are conducted: flexibility test, a ball drop test with a 227 g ball, a phantom drop test, a wear test and a cross-cut test. The definition of a suitable test method for assessing abrasion proved to be particularly complicated. Abrasion is measured on standard glass windshields using an abrasion roller. This unit, whose rollers are fitted with abrasive media, rotates on a circular path over the windshield’s glass surface. After a fixed test duration the scattered light on the glass surface is measured. A certain limit value must not be exceeded if the windshield wishes to achieve homologation.
This test method proved to be useless for plastic windshields because during tests in various laboratories it was discovered that the deviations between the results of the tested plastic materials were simply too great. At this point, three new test methods were proposed, and a combination of them proved to be successful in reflecting the abrasion properties of plastic windshields. These three tests are a washing test based on ISO 20566, a modified sand drop test based on UN R22 and a wiper test with a clearly defined number of cycles. The manufacturers can choose between the method used in the past or the new three-part test method for the certification of the plastic windshield.
Wiesbaden Fire Service (Germany) conducted a series of cutting tests to investigate the properties of plastic windshields in the event of an accident. The tests showed that it takes an almost identical length of time to cut glass and plastic windshields. The Fire Service came to the conclusion that the plastic windshield does not present an obstacle to a fast rescue in the event of an accident.
In another test project, the effects of the plastic windshield on pedestrian safety were studied. Impact tests were carried out as described in UN Regulation 127. This test method requires the impact of an adult head test model on a real windshield. Plastic windshields with a range of thicknesses were tested in direct comparison to a glass windshield on the basis of four impact positions. The head injury criterion (HIC), which assesses the head injuries caused by acceleration was used for the analysis. Although the impact of an adult head test model on a plastic windshield produced a higher HIC value than glass windshields, the defined limit value was only exceeded by an 8 mm plastic windshield – and only if the impact point of the adult head test model is in the direct vicinity of the bottom windshield frame. Plastic windshields have pronounced elastic properties and did not fracture in any of the tests.
The spread of plastic windshields will depend on the development of surface coatings for them. The surface coatings for plastic windshields must be resistant to abrasion from the wiper blades and stone strikes. The use of plastic materials can also reduce weight. However, a different shape and the ability to glue the windshield into position is a requirement for this. To date, there is a conflict between static and dynamic stability requirements for plastic windshields and their high energy absorption in the event of an accident involving a pedestrian.