Omar Khalid
The effects of paint on plastics are complex and multifaceted. Painting plastic can have a significant impact on its mechanical properties, including strength and stiffness. The interaction between paint and plastic is intricate, and the addition of paint can sometimes lead to reduced impact strength and increased brittleness in certain types of plastics. On the other hand, in some cases, painting has been observed to increase the toughness and elongation of specific plastic blends. This complexity underscores the necessity of thorough testing to understand the unique behaviour of different plastics when painted. The process of painting plastic parts can influence their durability, and it is essential to ensure that any paint applied does not compromise the integrity of the plastic material. Understanding the effects of paint on plastics is crucial, especially when considering the conservation of plastic items in museums and collections, where embrittlement over time can pose a significant challenge.
| Characteristics | Values |
|---|---|
| Effect of painting on plastic | Painting can affect the mechanical properties of plastic parts, such as impact strength and tensile strain at break |
| Factors influencing paint's effect on plastic | The type of paint, the type of plastic, the presence of adhesion promoters or topcoats, and the combination of these factors |
| Impact on impact strength | Painting reduced impact strength in three ABS and ASA resins, but not in ASA with a topcoat without an adhesion promoter |
| Influence on tensile strain at break | Painting increased tensile strain in the nylon 6/ionomer blend, but reduced it in other materials |
| Stiffness and strength of paint and plastic | Paint is often more brittle than the plastic substrate, which can lead to cracking under normal loads for the plastic |
| Reversibility of plastic embrittlement | Embrittlement can be reversed by remelting the plastic and adding new plasticizers |
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What You'll Learn
Paint can embrittle plastic
Painting plastic can have a significant effect on its mechanical properties. The strength and stiffness of the paint are different from those of the plastic, but they have to move in conjunction. The paint is often more brittle than the plastic substrate, which means that it will crack under loads that would not normally crack the plastic. Once the paint starts to crack, it can either continue into the substrate, cause the paint to delaminate from the substrate, or channel through the paint without affecting the substrate. The outcome depends on the difference in stiffness between the paint and the plastic, as well as the strength of their bond.
The complex interaction between paints and plastics is illustrated by BRP's approach to the Evinrude E-TEC G2 outboard motor, which used a mix of painted and molded-in-color plastic motor covers. Testing revealed that painting reduced the impact strength of three ABS and ASA resins. Notably, the G-P ABS became severely embrittled when painted with a topcoat only.
However, the effect of painting on tensile strain (elongation) at break varied. Painting reduced the strain at break for three out of four tested plastic substrates, causing them to become more brittle. Interestingly, the nylon 6/ionomer blend behaved differently, showing increased elongation after painting.
The plasticizer added to ABS plastic to make it softer can evaporate over time, causing the plastic to become brittle again. This issue can be resolved by remelting the plastic and adding new plasticizers, which have improved to be less volatile.
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Paint is often more brittle than plastic
Painting plastic can have significant effects on its mechanical properties. The strength and stiffness of the paint are different from those of the plastic, but they have to move in conjunction. Often, the paint is more brittle than the plastic substrate, which means that the paint will crack under loads that would not normally crack the plastic.
Once a crack in the paint starts, it can either continue into the substrate, cause the paint to delaminate from the substrate, or "channel" through the paint without affecting the substrate. The path taken depends on the difference in stiffness between the paint and the plastic and the strength of the bond between the two.
At the Society of Plastics Engineers' ANTEC conference, results were presented that demonstrated the effect of painting on four different plastic substrates: general-purpose ABS, high-impact ABS, weather-resistant ASA, and 10% talc-filled nylon 6/polyethylene ionomer. The results showed that painting reduced the impact strength of three ABS and ASA resins. However, the toughness of the nylon 6/ionomer blend increased after painting.
The effect of the topcoat on the general-purpose ABS was particularly notable. Unpainted, the general-purpose ABS had 22% strain at break. After painting with just the topcoat, the general-purpose ABS became very brittle, with just 5% strain at break. On the other hand, if a clear adhesion promoter was applied before the topcoat, the strain at break was 17.5%, not too different from the unpainted material.
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The strength and stiffness of paint differ from plastic
Painting plastic can have a significant effect on its mechanical properties. The strength and stiffness of paint differ from those of plastic, and understanding these differences is crucial when considering the application of paint on plastic.
The strength of a material refers to its ability to withstand stress without breaking when stretched or pulled. The stiffness of a material, on the other hand, is described by its flexural modulus, or its ability to bend. In the context of paint and plastic, the paint is often more brittle than the plastic substrate. This means that the paint may crack under loads that would not typically crack the plastic.
The difference in stiffness between the paint and the plastic, along with the strength of their bond, determines the outcome when the paint cracks. There are three potential scenarios: the crack can propagate into the plastic substrate, causing the paint to delaminate from the substrate, or it can "channel" through the paint without affecting the substrate.
The complex interaction between paints and plastics is further illustrated by the impact of painting on tensile strain (elongation) at break. Painting a nylon 6/ionomer blend, for instance, increased its strain at break, while painting other materials reduced their strain at break. Similarly, the impact strength of general-purpose ABS decreased significantly after painting, making it brittle.
In summary, the strength and stiffness of paint differ from those of plastic, and these differences can lead to various outcomes when cracks form in the paint. While paint can provide protection and enhance the aesthetic appeal of plastic, thorough testing is necessary to understand the complex interactions between paints and plastics fully.
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Paint can affect plastic's tensile strain
Paint can have a significant effect on the mechanical properties of plastic parts. The strength and stiffness of the paint are different from those of the plastic, but they have to move in unison. The paint is often more brittle than the plastic substrate, which means that the paint will crack under loads that would not normally cause the plastic to crack. Once the paint cracks, it can either continue into the substrate, cause the paint to delaminate from the substrate, or "channel" through the paint without impacting the substrate. The outcome depends on the stiffness difference between the paint and the plastic and the strength of their bond.
The impact strength and the strain at break are the properties most influenced by painting. In some cases, painting slightly reduces the tensile modulus and/or the tensile strength, but not by more than 10%. The strain at yield remains largely unaffected. It is important to note that these results are specific to the paint system, adhesion promoters, and plastics used in the tests. Different paint systems and resins may yield different results.
The test results highlight the complex interactions between paints and plastics. For instance, neither the clear adhesion promoter nor the topcoat significantly impacted the ASA material's impact strength individually. However, using them together reduced the impact strength of the ASA by 40%. The nylon 6/ionomer blend stood out, exhibiting increased elongation after painting, while the other materials showed reduced elongation.
The topcoat's effect on general-purpose ABS is noteworthy. Without paint, the general-purpose ABS had a strain at break of 22%. After painting with just the topcoat, the ABS became brittle, with a strain at break of only 5%. The G-P ABS was similarly affected, becoming severely embrittled with just a topcoat. However, when painted with both a clear adhesion promoter and a topcoat, it showed little loss of toughness.
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Plasticizers can prevent plastic from becoming brittle
Painting plastic can have a significant effect on its mechanical properties. The paint's strength and stiffness differ from those of the plastic, but they must move in unison. The paint is often more brittle than the plastic substrate, causing it to crack under loads that wouldn't normally affect the plastic. Once the paint starts cracking, it can continue to crack and affect the substrate, cause the paint to delaminate, or channel through the paint without impacting the substrate.
The impact of painting plastic was demonstrated at the Society of Plastics Engineers' ANTEC conference in Las Vegas. Results showed the effect of painting on four different plastic substrates: general-purpose ABS, high-impact ABS, weather-resistant ASA, and 10% talc-filled nylon 6/polyethylene ionomer. Painting reduced the impact strength of three ABS and ASA resins. However, the toughness of the nylon 6/ionomer blend increased after painting, and it showed increased elongation.
BRP's 'mass customization' approach to the Evinrude E-TEC G2 outboard motor resulted in a mix of painted and molded-in-color plastic motor covers. Testing revealed that painting reduced the impact strength of the G-P ABS with a topcoat by a significant margin. However, when painted with both a clear adhesion promoter and a topcoat, it showed little loss of toughness.
While painting can affect the mechanical properties of plastic, the specific effects depend on various factors, such as the type of paint, the plastic substrate, and the interaction between the two. Thorough testing is necessary to understand the complex relationship between paint and plastic fully.
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Frequently asked questions
Painting plastic can have varying effects on its mechanical properties. In some cases, painting can increase the toughness and elongation of the plastic. However, in other cases, painting can cause the plastic to become more brittle. The effect of painting depends on the specific type of plastic and paint used, as well as the combination of the two. Therefore, it is challenging to provide a general answer to this question.
The strength and stiffness of the paint are important factors. If the paint is more brittle than the plastic, it will crack under loads that would not normally affect the plastic. The difference in stiffness between the paint and the plastic, as well as the strength of the bond between them, will determine whether a crack in the paint affects the substrate.
Yes, in one example, general-purpose ABS plastic became very brittle after being painted with just a topcoat, with a significant reduction in strain at break from 22% to 5%. In another example, styrenic copolymers became more brittle after painting, while the nylon 6/ionomer blend showed increased toughness and elongation.
To prevent plastic from becoming brittle, a plasticizer can be added when the plastic is hot and malleable. Older plasticizers, such as dinonylphthalate, were effective but volatile, causing the plastic to become brittle again over time. Newer and better plasticizers have been discovered that are less volatile and can help maintain the softness of the plastic.
