Plastics are synthetic or semi-synthetic materials made from polymers, known for their versatility, durability, and moldability. They are widely used across various industries, including packaging, automotive, and electronics.
Types of Plastics:
Thermoplastics: Soften when heated and can be remoulded, making them recyclable.
Examples: Polyethylene (PE), Polypropylene (PP), Polystyrene (PS), Polyvinyl Chloride (PVC).
Thermosetting Plastics: Harden permanently after shaping and cannot be remoulded.
Examples: Epoxy Resins, Phenolic Resins, Polyurethane.
Elastomers: Stretchable materials with rubber-like properties.
Examples: Natural Rubber, Silicone.
Bioplastics: Made from renewable sources, offering a sustainable alternative.
Examples: Polylactic Acid (PLA), Polyhydroxyalkanoates (PHA).
Proper identification of plastics helps in selecting the right ink, optimizing adhesion, and ensuring a durable print. Not all plastics are the same. some have smooth surfaces that make ink adhesion difficult, while others may be textured, affecting print clarity.
Methods to Identify Plastics:
Visual Inspection: Recognizing characteristics like rigidity, texture, and transparency.
Burn Test: A controlled burn test to observe the flame colour and smoke for plastic identification (though this should be done carefully).
Density Test: Submerging in water to check if the plastic floats or sinks.
Resin Identification Codes: Using plastic recycling symbols to identify the material.
Common Plastics in Screen Printing:
Polyethylene (PE):
Properties: A versatile plastic that can be used in many finished products through extrusion, injection moulding, and other processes. Two types of PE are available in market. High-density polyethylene (HDPE) is a rigid material while low-density polyethylene (LDPE) is very flexible.
Identification
– Appearance: PE is often semi-transparent to opaque and has a waxy or slightly slippery feel.
– Resin Identification Code: Recycling symbol with the number 2 (for HDPE) or 4 (for LDPE).
– Density Test: PE floats in water due to its lower density (0.91–0.97 g/cm³).
– Burn Test: PE burns with a blue flame and yellow edges and smells like paraffin.
Application:
LDPE: Used in plastic bags, stretch wrap, and squeeze bottles.
HDPE: Used in milk jugs, detergent bottles, and rigid containers.
Polypropylene (PP):
Properties: A light weight plastic that can be used in many moulding methods. It is known for its durability and is resistant to chemicals, moisture and fatigue.
Identification:
– Appearance: (PP) is usually a translucent to opaque plastic with a smooth surface, available in various colours. It can be rigid or flexible, depending on its formulation
– Resin Identification Code: Recycling symbol is number 5.
– Density Test: PP floats in water due to its lower density (0.9g/cm³).
– Burn Test: PP burns with a blue-yellow flame, produces waxy odour.
Application: Bottles, cap, food containers, lab equipment, pipes.
PVC (Polyvinyl Chloride):
Properties: PVC is a cost-effective plastic known for its bright colours, resistance to acids and alkalis, and durability against breaking. Available in both rigid and flexible forms, it is widely used in various applications. However, PVC is not suitable for food containers due to the presence of toxic additives used in its production.
Identification:
– Appearance: PVC can appear as either rigid or flexible, with rigid PVC typically being opaque and smooth in white or grey, while flexible PVC is often transparent or brightly coloured with a soft, pliable texture.
– Resin Identification Code: Recycling symbol is number 3.
– Density Test: PVC has a higher density (1.3–1.45 g/cm³). It will sink when placed in water.
– Burn Test: PVC burns with a yellow flame and produces green-tinted smoke due to the presence of chlorine. It emits a pungent smell similar to hydrochloric acid and chlorine.
Application: Signage, banners, automotive parts and vehicle wraps.
Polystyrene (PS):
Properties: Polystyrene (PS) is a lightweight, versatile plastic with excellent thermal insulation. It is clear, glossy, rigid, and resistant to acids, though it dissolves in acetone.
Identification:
– Appearance: Clear, hard, and brittle in its natural form; available in solid and foam forms.
– Resin Identification Code: Recycling symbol is number 6.
– Density test: Polystyrene typically floats in water because it has a lower density (1.04 g/cm³), especially Expandable PS foam.
– Burn Test: Burns with a yellow flame and produces black soot. Emits a distinct sweet smell.
Application: Used widely in packaging, food containers, disposable cutlery and insulation.
Acrylic (PMMA):
Properties: Acrylic or polymethyl methacrylate (PMMA), is a clear, glossy and highly transparent plastic often used as a lightweight alternative to glass. It offers excellent UV and weather resistance, making it suitable for outdoor applications, and is strong with good impact resistance, though more brittle than polycarbonate.
Identification:
– Appearance: Acrylic is highly clear and glossy.
– Resin Identification Code: Recycling symbol is number 7
– Density test: With density of 1.18–1.20 g/cm³ it is lighter than glass.
– Burn Test: Burns with a blue flame emitting a fruity sweet smell.
Application: Displays, signage, awards and windows.
Polycarbonate (PC):
Properties: Polycarbonate plastic is typically clear and highly transparent, with a smooth, glossy surface, often resembling glass. It has excellent optical clarity and can be color-tinted or frosted, though it’s naturally transparent. Polycarbonate is also known for its toughness and impact resistance.
Identification:
– Appearance: PC is very clear and highly transparent.
– Resin Identification Code: Recycling symbol is number 7
– Density test: With density of 1.20–1.22 g/cm³ it is similar to acrylic.
– Burn Test: Burns with a yellow flame and gives aromatic smell.
Application: Electronics housings, durable goods.
Acrylonitrile Butadiene Styrene (ABS):
Properties: It typically has an opaque, matte finish and is available in a range of colours. It has a smooth, rigid surface with a slightly glossy appearance and is known for its toughness and impact resistance. An engineering plastic that is easy to process and form.
Identification:
– Appearance: Smooth, rigid and highly glossy or matte
– Resin Identification Code: Recycling symbol is number 7
– Density test: It is slightly heavier than water, with density of around 1.04–1.06 g/cm³ and will generally sink.
– Burn Test: Burns with a yellow flame and creates black, sooty smoke. a strong, acrid smell similar to burnt rubber.
Application: It is used in durable products like electronics, televisions and calculators.
Testing plastics beforehand is essential to find the best match for your ink and printing process. Dyne pens can be used to check surface energy of substrates. Value above 40-42 is considered good for printing. Proper surface preparation is also vital, as oils or residues can compromise adhesion and lead to ink chipping or peeling. Various surface treatments enhance adhesion, durability, and appearance, improving bonding with inks and coatings.
Surface Treatments:
Three main types of surface modification are:
Flame Treatment: Briefly exposing the plastic to an open flame to increase surface energy and improve ink adhesion. Common for packaging materials like PP and PE.
Corona Treatment: Using electrical discharge to treat plastic surfaces for better ink adhesion.
Chemical Primers: Coatings that enhance bonding on challenging plastics by applying a chemical solution to improve surface printability and adhesion.
Plasma Treatment: Ionized gas creates bonding sites on surfaces, effective for plastics like polypropylene and used in automotive and medical fields.
Lesser-known methods are Chemical Etching, UV Treatment and Mechanical Roughening.
Understanding plastic types, applications, and surface treatments is key to achieving high-quality, durable screen prints. Selecting the correct ink type and performing appropriate surface treatments ensures optimal results on different plastic materials.
Article by Dr. Mustafa Kapadia