Fig 1 shows a transparent protective cover for a desk lamp. The cover is made from acrylic (PMMA) using vacuum forming. (a) Give two properties of acrylic (...

(a) Two properties of acrylic (PMMA) that make it suitable for a transparent lamp cover:

1. Transparent / optically clear - acrylic transmits up to 92% of visible light, allowing the lamp to illuminate effectively through the cover. [1]
2. Rigid / stiff - acrylic holds its shape under normal use, providing structural protection for the lamp without sagging or deforming. [1]

Other valid properties include its good surface finish (can be polished to restore clarity), its lightweight nature, and its resistance to UV degradation.

(b)(i) The vacuum forming process used to produce the lamp cover works in four key stages:

1. A flat sheet of acrylic is clamped securely in a frame above a mould (the mould is shaped like the desired lamp cover). [1]
2. The sheet is heated by an overhead radiant heater until it becomes soft and pliable, typically reaching 160-180 °C for acrylic. [1]
3. The mould is raised up into the softened sheet from below (or the sheet is lowered over the mould). [1]
4. A vacuum pump removes the air from beneath the sheet through small holes in the mould, creating a pressure differential that pulls the softened acrylic tightly over the mould surface. The formed shape is allowed to cool and harden, then removed from the mould and trimmed to its final size. [1]

The process relies on atmospheric pressure (approximately 101 kPa) pushing the heated sheet onto the mould once the air beneath is evacuated.

(b)(ii) The four key parts of the vacuum forming equipment that should be labelled on the diagram are:

1. Clamp / frame - holds the plastic sheet in position. [1]
2. Plastic sheet / acrylic sheet - the thermoplastic material being formed. [1]
3. Mould / former - the shaped tool over which the sheet is drawn. [1]
4. Platen / base with holes - the platform containing small holes connected to the vacuum pump. [1]

The heater element is already labelled on the diagram. The vacuum pump connection at the bottom draws air out through the holes in the platen.

(c) The fundamental difference between thermoplastics and thermosetting plastics lies in their molecular structure:

• Thermoplastics have polymer chains held together by weak intermolecular forces (van der Waals forces). When heated, these weak bonds break, allowing the chains to slide past each other so the material softens and can be reshaped. On cooling, the bonds re-form and the material becomes rigid again. This process is reversible and can be repeated many times. [2]
• Thermosetting plastics undergo an irreversible chemical reaction during their initial forming (curing). Strong covalent cross-links form permanently between adjacent polymer chains. Once cured, these cross-links do not break when reheated; instead the material will char and decompose rather than soften. The shape is permanent and cannot be reformed. [2]

(d) Comparing thermoplastics to thermosetting plastics:

• Advantage: Thermoplastics can be recycled, reshaped and reformed by reheating, reducing waste and allowing off-cuts to be reused. [1]
• Disadvantage: Thermoplastics have lower heat resistance than thermosets and may soften or deform if exposed to high temperatures near the lamp. [1]

(e)(i) A plasticiser increases the flexibility and workability of a polymer. [1] It works at the molecular level by inserting small molecules between the long polymer chains, spacing them further apart. This increased spacing allows the chains to slide past each other more easily, making the material softer and more pliable. [1] A common example is the addition of plasticiser to PVC: rigid (unplasticised) PVC is used for pipes and window frames, while plasticised PVC is flexible enough for cable insulation and clothing.

(e)(ii) One other type of polymer additive:

• Name: UV stabiliser [1]
• Purpose: A UV stabiliser absorbs or blocks ultraviolet radiation from sunlight that would otherwise break down the polymer chains, causing the material to become brittle, discoloured or degraded over time. [1]

Other valid additives include fillers (add bulk and reduce cost), flame retardants (reduce flammability), and antioxidants (prevent oxidation degradation).

(f) Restoring the transparent finish of scratched acrylic involves two stages:

1. Use progressively finer grades of wet-and-dry abrasive paper (starting from around 400 grit up to 1200 grit) to carefully remove the scratches from the surface. [1]
2. Apply a polishing compound (such as Brasso or a specialist acrylic polish) using a soft cloth or buffing wheel, working it into the surface until full optical clarity is restored. [1]

(g) Thermoplastics can be recycled but thermosetting plastics cannot, because of their different molecular structures:

1. Thermoplastics have weak intermolecular forces (not permanent chemical bonds) between their polymer chains. [1]
2. When heated, these weak forces break, allowing the material to melt and be reformed into new products. The chains can re-bond on cooling, so the recycling process can be repeated. [1]
3. Thermosetting plastics have permanent covalent cross-links between their polymer chains. These bonds do not break when reheated; the material decomposes rather than melting. Because the molecular structure cannot be reversed, the plastic cannot be reshaped or recycled by remelting. [1]